I changed my strings'gauge from Meduim to Extra Light , Do i have to Adjust the saddle and the strings height for lower ? i mean if i'm using extra light gauge , what string height is right for me and what to do with the saddle ?
Extra light strings will tune to pitch under much lower tension and therefore have a wider oscillation envelop when they vibrate. For that reason it is often necessary to raise the action when switching to lighter strings - and one way to raise the action is to raise the saddle height by putting a shim under it (on acoustic guitar). However, it is not always necessary, depending on the original height of your action.
EDIT: If you do ever find a need to adjust your saddle height - you can refer to this link Adjusting Saddle on Acoustic Guitar
It may also be desirable to make a truss rod adjustment when switching to different gauge strings. Lighter strings will put less tension on the neck so less counter tension will be needed from the truss rod.
The other thing that could need adjusting if you intend to always use lighter gauge strings (permanent switch) is the width of the nut slots which may be too wide for the lighter strings.
Having said all of the above, it is possible that your guitars action was set in a way that was able to handle the lighter gauge strings with few or no adjustments required. The neck may be laying a little flatter without as much tension - which would make your action lower.
A simple test to determine if any adjustments to the truss rod is needed, simply play each string at each fret with a normal plucking force (not too hard not too light) and look for fret buzz. Fret buzz (where the oscillating string is contact the frets while vibrating) would indicate the action is too low. If you do get some fret buzz I would start by checking the relief in the neck and consider loosening the truss rod. If the neck is too flat (no relief) or back bowed (negative relief) you will definitely need to loosen it. But if you can play buzz free all the way up the neck - you are all set with no adjustment needed (unless you have room to lower the action because there is still too much relief - not likely).
If you do find that a truss rod adjustment may be indicated - a skilled professional guitar tech or luthier can do this for you and check the rest of your set up. However, if you are the do it yourself type and are very careful, with the proper truss rod adjustment wrench, you can do it yourself.
If you decide to attempt a truss rod adjustment on your own - first be sure you have the exact correct truss rod adjustment wrench (one may have come with your guitar if purchased new). It should be a very snug fit with no wiggle. You don't want to strip the truss rod by using one that is too loose.
Then refer to many tutorials on line such as this one - How to adjust your truss rod on acoustic guitar. Here you will learn all the important caveats like "don't over tighten", "make changes slowly" etc. - as well as ways to tell if your neck has the proper relief or not.
Once you are able to get buzz free playing from your lighter strings, you will find your guitar much easier to play. Good luck and enjoy your "new" guitar!
1some people say that truss rod adjustment can ruin the guitar.– SomeoneApr 28, 2016 at 20:45
1@Eva - If not done correctly it can absolutely ruin your guitar. That's why I recommend a pro. But if you do it yourself, read about all the precautions on the tutorials available on line. The one I linked appears to be quite comprehensive. You will also find tutorials on YouTube. Don't attempt to do it yourself if after reading all about it and watching a vid - you don't feel comfortable. One guitar I purchased new actually came with instructions for adjusting the truss rod. I do mine whenever needed. I leave saddle work to the pro's. Apr 28, 2016 at 20:53