External
This will still erase the internal OS, you have been warned
For using it over the external sd slot, you of course need to have another micro sd card, with a size of at least 4GB.
If you don't have another micro sd card, then your only option is to use the internal option.
You also need to own an SD card usb adapter or something similar so you can write files to it.

Download both the firmware file (according to your hardware revision, behind the battery) and the external tool.

Unzip ExternalRS97_patcher.zip and put the folder patcher_ext on a micro sd card.
Plug that micro sd card into the RS-97, go to the Folder icon and look for your "patcher_ext" folder.
Once you found it, run the file run.dge. Screen will become black for a while, wait until it goes back to where it was.
Once the screen is no longer black, you can reboot by running reboot.dge. If it becomes stuck, then hit the reset button or remove the battery.

Now, you need to flash the firmware to another micro sd card. (You can reuse the one you used for the patcher, it's no longer needed)
Format said micro sd card with this tool :
https://www.sdcard.org/downloads/formatter_4/

Then use https://etcher.io or Win32 Disk Imager to write the firmware file to the Micro sd card.
Once that is done, you can expand your FAT32 partition with a tool like Minitool or Gparted.

Put that micro sd card in the GBA slot. Make sure your console is powered off !
Then power it on with the micro sd card inside the GBA slot and it should boot it.