Go on the map selection screen, then back to the main menu, then press 0 (not on the numpad) to get 100k XP on every tower as well as your account.
If the nexus version doesn't work, download it from here: https://github.com/Timotheeee/btd6_mods/blob/master/infinite_xp/infinite_xp.dll?raw=true
installation guide: https://hemisemidemipresent.github.io/btd6-modding-tutorial/
code: https://github.com/Timotheeee/btd6_mods/blob/master/infinite_xp/Main.cs
If the nexus version doesn't work, download it from here: https://github.com/Timotheeee/btd6_mods/blob/master/infinite_xp/infinite_xp.dll?raw=true
installation guide: https://hemisemidemipresent.github.io/btd6-modding-tutorial/
code: https://github.com/Timotheeee/btd6_mods/blob/master/infinite_xp/Main.cs