Actually, there may be several reasons why the string height changes on your accoustic guitar. Most commonly it's because something has happened to the neck.
Most of the times these issues can be corrected with the truss rod inside the neck. If you don't know what your doing, then I suggest you take your guitar to a shop or guitar repairer and have someone look at it. If it's just the truss rod, then it is very easy to fix.
Think of your guitar neck taking on the form of a wide and shallow "U" or a "valley". This often times happens because you change to a heavier set of strings, or because of natural changes to the wood in the neck.
By tightening the truss rod (just a tiny fraction at the time!), you will usually compensate for this.
Some other reasons why the string height changes is because the bridge is loosening from the top. This can be fixed, but you will need someone who definitely knows what they're doing.
Then the guitar top itself may have become bowed or "bumped" - raising the strings further from the fret board. Some times you have a loose brace underneath the guitar top adding to this problem. A guitar top being bowed is almost impossible to fix.
Just to make it clear - some of these issues occur because people put steel strings on guitars built for nylon strings, or use way too heavy strings on a more delicate guitar.
The way I see it, you should take your instrument to someone who can have a look at it for you.
Good luck!
Beginner Guitar Lessons
