You can safe your project via File - Save. You can't name it yourself, the existing document will be updated.
Via File - Snapshot you can save a copy of your project. ATLAS.ti will propose a name for this copy, but you can change that. After the copy has been saved, you'll be back in the "existing" document (check the displayed project name).
Please note: this is different from using the "Save as"-function in for example Word!
A project that has (only) been saved via File - Save, can't be opened on another device!
You have to safe it in another way to be able to open and use the project on another device.
See Creating Creating a Project Bundle maken for more information.
ATLAS.ti can be a sensitive programm regarding saving, updates on your system, etc.
Because it is better to be safe than sorry, we advise you to regularly create back-ups of your project and to store them on a different location than your own pc.
Create a Project Bundle as mentioned above. Make sure you give the Project Bundle a recognisable name (think of the data, version, etc.) and that you safe it somewhere you can find it back easily.