1. "Save As...", give your map a new name.

2. Goto...
C:\Program Files (x86)\Steam\steamapps\common\Men of War Assault Squad 2\mods\the war machine make a map\resource\map\single\The War Machine MaM\700x300_your_map

...or the 600x400_your_map respectively.
Copy the o.mi file and place it in the map folder that would been created at step 1, located at...
C:\Program Files (x86)\Steam\steamapps\common\Men of War Assault Squad 2\mods\the war machine make a map\resource\map\single\The War Machine MaM

3. Goto...
C:\Program Files (x86)\Steam\steamapps\common\Men of War Assault Squad 2\mods\the war machine make a map\localization\mission\single\the war machine MaM

Copy and paste either 700x300_your_map.lng or 600x400_your_map.lng and rename the copy to match the filename created at step 1.

4. Open new .lng file with a text editor (eg. notepad) and find the line...
{"name" "700x300_your_map"}
...it is near the bottom.
Edit it to reflect the name of your map.

5. Have fun