There are no tutorial online for changing such strings.Are those small parts at end of string detachable
Given that those strings are the same as the originals, they are supplied with the stop at one end. It's there to stop the string pulling through the hole in the bridge. However, if you have strings with no stop, they're probably nylon, and will need a wrap around the bridge, as in classical guitars. I wouldn't have thought a tutorial was necessary, as taking them off is the reverse of fitting new ones.
It is in fact a steel string guitar, it just has an unusual bridge for a steel string. Those "small parts at end of string" are called ball ends and you do not want to "detach" them (the only way to do that would be to cut them off, but don't do that). You need the ball ends for this to work. If you look at my picture, you can just barely see the ball ends still attached to the strings at the bridge.
I'm not sure exactly what is causing you trouble about changing these strings. The picture in your question shows the correct way to run the strings through the bridge. The ball ends are there to hold the strings into the bridge. The part in your picture is actually the easy part of restringing this guitar. The harder part is at the other end where the tuning machines are.
There are many instructions online for changing strings. One example is here. If you follow that link, you probably want to start at step 4. Steps 1 - 3 are for a different kind of bridge from the one you have, which is why it looked like you might actually have a nylon string guitar, because your bridge is unusual.
I don't do exactly the same thing as those instructions, but they will work fine. It's just personal preference. I included that link because there are pictures which will probably help a lot. Here's what I do:
- Run the string through the hole of the appropriate tuning machine, pulling the string until there's hardly any slack (for now).
- With my right hand, I gently pinch the string right where it passes through the tuning machine hole to hold it there. With my left hand near the sound hole, I pull the string back a bit to create some slack. I put more slack in the higher strings and less slack in the lower strings. I might pull the low E string about 3" to the side, and the high E string I might pull 8" or more to the side to put slack on it.
- With my right hand still pinching the string, I tight up my pinch while my left hand starts to turn the tuning machine as if I'm tuning the string up in pitch. This should make the string bend around the inside edge of the tuning peg. My right hand pinches to bend the string around the edge of the hole and around the tuning machine. I bend the string in two places on both sides of the whole.
- I wind the string 100% below the hole in the tuning machine. I don't put any windings above the whole. I use my right hand to guide the string so that it winds from just below the hole and then the next wrapping below that and so on down the peg, while my left hand keeps turning the tuning machine.
- I keep tightening until the string is just barely tight enough to make a sound and the string is seated correctly in the saddle and the nut.
- I repeat all that for all the other strings, going from low E to high E.
- When I'm done putting on all the strings, I tune them up with an electronic tuner and play them a bit, bending them a little to get them to seat and stretch (I'm not convinced they stretch at all but they definitely seat themselves). Then I tune again. I do that 3 or 4 times and then I cut the excess string ends from the tuning machine side (carefully to avoid cutting a string) with a set of serious wire cutters or guitar string cutters. Guitar strings are tough steel and can damage softer cutting tools like fingernail clippers or scissors (learned that the hard way).
The thread binding on the problem strings seem to be causing the strings to bind when passing through the holes.
Either remove the binding or buy new string without bindings.