mrdollsteak once said on reddit that the best way to learn hex editing at the level it is was to crack open a tutorial and just follow it through
here, he explains how to change the move in a tm with a small intro on hex editing, what it is and what you are doing
he then linked
this tutorial as his example of one to crack open and just go through
now, i personally use hxd, but others use like hex workshop or even gold finger. the important part is to stick with whatever you choose and get comfortable with it
also, as you are hacking emerald, i can not recommed enough that you follow
this tutorial to change advancemap's ini file so that it doesn't absolutely destroy your rom. emerald used to be considered "unusable" because of stipulations on free space and roms getting their sound corrupted because advancemap is programmed to overwrite music data as-is. it's actually quite sad
to answer your question, a little bit of explanation is necessary... emerald only contains the normal forme (encountered in the wild) and the speed forme (used for every deoxys you own)
fire red gets attack forme in the same manner
leaf green gets defense forme in the same manner
and ruby & sapphire get the normal forme, unedited by code
this post by kleenexfeu has the hex edit to change deoxys to normal forme in pokemon emerald. it essentially just takes the asm (code) commands there (which change every deoxys you own into a speed forme deoxys) and changes them to "nop," which is the command which tells the gameboy to do nothing and move on
because all of the deoxys formes aren't even present in emerald, there is essentially no way to edit them
