When you first launch the editor, you will have Autosave automatically enabled. This will save your map at a chosen timed interval. You can set how often it will automatically save by clicking the Left Mouse Button (LMB) on the arrow on the bottom-right of the Autosave Interval and choosing the time you want, as shown in the following screenshot:
You will find the Autosave feature at the bottom right of the editor. If you enable Autosave, there are a few options such as Interval and Type.
Save manually by going up to File | Save As.