You dont. There is a de-compiler, but dont use them (illegial unless the createor sent out the source code). Find the .sma of the file to edit them. If this is evm/eea they dont release the .sma, so its kinda crazy for you to think that way.
If you have the .sma , use a text editor (notepad) to edit it. The code for a .amx is almost unuseable.