You may change the map the server starts up with in the config file as mentioned before.

For each map no the server, there is a map file (.vxl) and always a map config file (.json). For example for ArcticBase there is ArcticBase.vxl and ArcticBase.json.

Map file (.vxl) contains the map data. Config file contains some configuration such as spawn points and map format.

mapType property, in the map config file, defines how the .vxl map should be loaded.

Map sizes by mapType
- legacy_aos_v1 for 512x64x512
- steam_aos for 512x256x512
- steam_aos_v2 for 500x256x500

Right now I won't bother to document map config files more in depth because they're subject to change. You may find plenty of example configs in StreamingAssets/Maps when you install the server.