You can't edit them. The *.amx files are after compiling (like dll's) so You can't edit them. But You can edit a source (*.sma) of every *.amx plugin and then compile Your-self. I guess You need change or add something in some existing AMX (or AMX X) functions. But for this a better way for You is ask at AMX (or AMX X) forum about this:
http://djeyl.net/forum/index.php?act=SC&c=7
http://www.amxmodx.org/forums/