Often there's a pin-hole you push in with a pen or a needle and release it at the location of the new combination. You may have to know the old code in order to be allowed to set a new one.
The combination for the glove box can't be changed until the glove box is opened. The factory code is "000". You probably have a used car with a different code set. The first step is to find the...
Contact the manufacture. There are several different brands of locks with several different versions for each brand. A correct answer besides this one from this website (or any website like it) is...
1. Open the lock. 2. Rotate the shackle 180 degrees so that it is parallel with the body of the lock -- just as though you were about to close it, but with the shackle facing the wrong way. 3. Push...