Can I choose which file is the main tex file in a project on Overleaf?

Yes you can—simply open the Project menu in the editor, click on the down arrow next to the file which you wish to select as the main .tex file for that project and click 'Set as Main File'.

Overleaf set as main file screenshot

You can do this as many times as you like in a project, so if at a later date you'd like to choose a different file to be the main file, simply repeat the above on the new file.