The simplest way to edit a config file is to use samba shares. It should be enabled by default. The wiki explains it all.
Alternatively you can use a FTP client like WinSCP (check the section above Samba shares in the wiki).
You can also use SSH to edit config files. (wiki link: https://github.com/retropie/retropie-setup/wiki/ssh
The command you can use to edit the config file would be
sudo nano /opt/retropie/configs/zxspectrum/.zesaruxrc
Place your roms in /home/pi/RetroPie/roms/zxspectrum folder. ZEsarUX supports .P, .81 and .Z81 formats for the ZX81.
Once you’ve installed ZEsarUX, there will be a +Start ZEsarUX script. You can use this to launch the emulator and have a play with a ZX81 computer. You can also load tapes using Smart load option from the menu (F5).
Roms should automatically appear in ES. If they do not, then it’s because the format isn’t included in es_systems.cfg and you will need to edit the file. Let me know if you’re not sure how to do this.