First of all, what does it mean when someone says a guitar is warped? Well, it is actually referring to the neck of the guitar. The acoustic guitar is made of wood, and wood can be warped due to humidity and weather. Imagine what would happen if you leave your guitar in the baking sun? Not only will the strings expand, but the wood will start to bend too. When this happens, we say that the guitar is warped.
So, how do you know if your guitar is warped? Well, have you ever experienced tuning your guitar correctly, only to find that it sounds terribly out-of-tune when you play a chord? This is a tell-tale sign that your guitar could possibly be warped.
Another way to check if your guitar is warped is to play the first string for example, and then play the 12th fret of the first string. This should give you a sound that is one octave higher. So, they should sound the same (only that the sound on the 12th fret is one octave higher). If the 2 notes do not sound the same, then your guitar is probably warped.
So, how do you "de-warp" or rather, fix a guitar that is warped? I would suggest to bring it to the guitar shop and have them fix it for you. However, if you are feeling confident enough, you could attempt to fix it yourself (not recommended unless you know how to do it).
From what I know, you will have to adjust the truss rod of your guitar with a type of allen key. (Here's a picture of how a truss rod looks like. It is in your guitar.)
Then, you have to locate the truss rod and use the allen key to straighten it. Some guitars have it at the neck, and some have it in the body. Here are pictures on where the truss rod can be located:
Yup, so after you have located the truss rod, you have to attempt to straighten it by inserting the allen key and rotating it till it's straight.
I have not done this before, so I wouldn't recommend to try this unless you really know what you are doing.