If you want to switch to another map, you have several options:
You can find an overview of all existing maps of a game world by clicking on the "Maps" link under the minimap or by clicking on the "Maps" tab at the top of the game menu.
Sending units to other maps
If you want to send units to other maps to start an invasion, you can do this at any point on the edge of your map. Select a unit and click on the turf at the edge of your map, then the message to switch maps opens, then you can choose which map you want to switch to.
Switching to other maps requires a lot of fuel, how much exactly is always displayed. For vehicles + helicopters, it is 5000 times the required fuel per second. For airplanes it is 1000 times. 40% of the fuel used is credited to the Destination Card so that you have enough fuel on the other card to move.
Moving with Homebase
To select a new home map and thus switch from your previous map to a new one, click on the "Setup" tab in the Games menu and then on "Game Settings". Afterwards you see the point "Himebase Map". After clicking on the drop-down menu you can select a map. Please note: You can only change your main card if you do not own more than one base (= only the homebase can be transferred to another card).