No problem. Stick your 3DS SD card into your computer. Go to mGBA folder. Inside this folder, there should be a config.ini file.
[SD Card drive] > mGBA > config.ini
You can open this file and edit it with Window's Notepad or any text editor, but it's kinda bundled up. The preferred program is Programmers Notepad but not needed.
- Open config.ini with Notepad
- Press Ctrl + F to open up Find
- type in
[ports.3ds] , Find Next
- right after the
] character, type in your bios path
for example, mine is:
bios=/mGBA/bios/gba_bios_Normmatt.bin
, which has a subfolder called
bios inside the folder
mGBA
My chosen bios is
gba_bios_Normmatt.bin
Make sure you do not leave any spaces in what you see as a single run-on line, and that the 'bios=/mGBA/...' entry is the only in there.
Edit : if it's too confusing on Notepad, get Programmer's Notepad here:
http://www.pnotepad.org/
When you install mGBA, the app is supposed to create that file. If you don't have one, I can share you mine.