To load a different terrain for offline play or for hosting a LAN session, click Options->Arena Setup->Environment->Change Terrain from the clipboard menu.
Terrains are stored in your ../HTC/Aces High II/terrains directory with a file extension of .res. The dialog lists all terrains present in your terrains directory.
Whenever HTC releases a new terrain, or if you join a LAN session with a custom terrain, the terrain is automatically downloaded to your terrains directory. Any terrain present in your terrains directory may be loaded for offline practice.
If you have created a custom terrain and loaded it for use in a LAN session, anyone joining your session will automatically download your terrain from your computer to theirs.