You want to change the emulator and the core for one or more games. Here's the procedure to follow :

Select the games you want to change the emulator/core (highlight them) and click on the ' Reset Core / Emu ' button. A window appears allowing you to choose from a drop-down list the emulator and the desired core.

Validate the choices by clicking OK. You can reset by clicking on the checkbox ' Delete Emulator ' and ' Remove Core '. In this case the emulator and core sets for the system will be used.

This operation adds or updates, in the gamelist.xml, the tags <core> and <emulator> . These tags will be written during the next backup of the gamelist via the button ' Create gamelist.xml for roms '

Emulators and core can evolve depending on the versions of the software Recalbox and Batocera. You can update the list of emulators and core, by clicking on the link 'Editer liste Emulateur / Core' which will open the file '%appdata%\roaming\nexouille soft\database\emulator_core_batocera.txt' for Batocera or '%appdata%\roaming\nexouille soft\database\emulator_core_recalbox.txt' for Recalbox.

The file looks like this :

auto;auto
amiberry;auto
amiberry;A500
amiberry;A500P
amiberry;A1200
amiberry;A4000
amiberry;CD32
citra;auto
citra;citra
dolphin-emu;auto
dolphin-emu;dolphin-emu
dos;auto
dos;dos
fba2x;auto
fba2x;fba2x
fsuae;A1000
...