How do I change the map on my Unturned server?

Map is a setting on the Startup page, not a config file you have to go and find. Pick the map you want, restart the server, and it loads on the next boot.

The maps that ship with the game are PEI (the default), Washington, Yukon, Russia, Germany, Alpha Valley, Monolith, Destruction, Tutorial and Paintball_Arena_0.

The important thing to understand is that changing the map starts a fresh world on that map. It does not carry your existing base and players across. Your old world is not deleted, though: it stays on disk under the server's Servers folder, so switching back to the previous map returns you to the world you left there. That makes it safe to try a map and go back, but it does mean each map keeps its own separate save.

Because of that, take a backup before you switch if the current world matters to you. The Backups page captures the whole server including every map's saved world, so you have a copy you can restore from regardless of which map you end up on.