I am using
Guitar Pro 6. It has an option to decrease bpm of a song.
Is it ok to decrease a song's bpm and practice if you can't play at the song's original bpm?
YES! Of course. That's the best thing to do.
Every time you can't play a song at its normal bpm / speed (tempo), decrease the speed to a point where you feel comfortable with, and practice it there. After some practice, you'll be able to increase the bpm/ speed and after a while, you'll be able to play it at its normal speed.
This is good practice for your exercises as well. In a slower bpm, you'll be able to understand the mistakes you make, thus correcting them. If the exercise/song is fast, it's likely to make mistakes and not to realize them.
As @Bob said in the comments, it would be good to gradually increase to and then above actual bpm during practice. If you can play music above the actual bpm, it'll seem easy at the normal bpm.
Absolutely. There is no better way known to Man. It has always been the way. Folks these days are very lucky - they have the facility to slow tracks to whatever comfortable speed is needed, without changing the key. That's pretty important. When I was learning, we could slow the record player (remember them !) from 45 to 33, sometimes 16 rpm, but that entailed a change of key as well as tempo.
Make the most of these facilities !
Yes, This is exactly how you should be doing it! I have been playing the guitar for over a decade now and took lessons all the way through my middle school and high school years. When ever I was learning a new song, a new scale or a new finger dexterity exercise my tutors would set the metronome at a slow tempo so I could focus on proper finger positioning and proper fingering technique.
Once you start to feel comfortable with what ever you are trying to learn, bump up the speed so you are challenged with keeping everything perfect. After awhile that new tempo will become easy and you can bump it up again (just remember the new tempo should be challenging but not force you to make lots of mistakes). Soon you will be playing it at it's normal speed!
A few Suggestions, never stop increasing the metronome speed for scales and finger dexterity exercises, those things you can always improve upon. For actual songs, however, you might want to bump the speed up past the standard bpm for 2 reasons. 1) Practicing it faster makes playing it at the normal bpm easier, and 2) if you plan on playing in front of an audience your adrenaline and nerves might make it so you speed up due to stress (I know it did for me in the beginning). Being able to play it well and quickly at home will help you play well at the normal tempo when you're under the stress of being in front of a crowd.
This is certainly the traditional way to practice, but you need to be aware that this method may not lead to the gain of the technique needed to play faster passages.
Consider learning to run, for example - you cannot learn to run by just walking faster and faster, running is a fundamentally different technique.
To apply to the guitar, consider what would happen if you practice only moving your pick downward - no matter how much you practice, there will be a speed at which it is no longer possible to pick well, since you have never learned the technique of picking both on the way down and on the way up.
So, practicing slowly is fine to understand the exercise, and get it under your fingers, but it is also really important to watch carefully someone who knows what they are doing so that you can be sure that you have are using the correct technique - otherwise, you will become a very fast walker but never learn to run.
(This is coming from the perspective of a classical pianist, not a pop/rock guitarist, so bear with me.)
This is, as stated in some other answers, the correct, traditional way to practice. In many cases, the only way to gain the muscle-memory necessary to play fast passages is by repeatedly working them out very, very slowly. (Watch a video of, oh, say, Un Barque Sur L'Ocean by Ravel, and ask yourself whether the pianist could possibly have sight read the piece at that speed.)
There are, however, some other useful techniques for learning fast passages. The best technique I can recommend for general-purpose practicing is to work on small groups of notes (about 2-6 notes/chords at a time) at or above the performance speed (sometimes called "hyperspeed practice"). You should work on a single group (even if it's two notes!) until it's not just perfect every time, but easy to play perfectly every time. Then work on a couple more groups until you have a "group of groups" (i.e. a complete musical phrase or, if the passage is extremely complicated, perhaps only half a phrase or even just a single measure). Play this "couple of groups" at "hyperspeed", holding the last note of each group before going on to the next group. It's also useful to switch up which notes are considered part of the groups and which are held in between groups; you don't want to hesitate on the notes that you practiced as the "held" notes. (For example, say you have a run of 16th-notes; you could practice in groups of four, first stopping and holding the first note of every beat, then the second, then the third, then the fourth.) Once you've done this, play everything in rhythm (i.e. without holding notes longer or practicing groups in "hyperspeed") slightly below tempo, then bring the whole thing up to tempo.
No matter what tempo(s) you use when you practice, there are a couple things you must always be aware of; I'll try to list the most important ones here.
Note that practice makes permanent; poor practice habits will lead to poor playing, regardless of what tempo(s) you're practicing at.