FF6 Hacking

Hello guys, I want to change Terra sprite for the GBA, I don't like her look at all but I don't know how to start.
Please could someone give me a tutorial, Step by step. I am a beginner, so I don't know anything about this.
The reason, is because I want to have the best experience playing FF6, GBA has the content, but it is not so easily to change the sprites.
I don't like the SNES version because it has less content. 

currently there doesn't exist a sprite editor for gba. snes is the only choice

The only way currently to edit GBA sprites is to use something like YY-CHR and draw the tiles one by one with a reference sprite sheet. As an example, the character sprites start at $760000 in the (U) version. You could try loading a zsnes savestate in YY-CHR to get the correct palette and if the color order is the same on GBA, the palette would be accurate.

You could also try to convert a spritesheet to GBA 4bpp with this Gimp plugin or another utility and use two instances of YY-CHR, one with the converted spritesheet and one with the GBA ROM and copy/paste the tiles from one instance to the other.