Click on the three dots in front of the file's title.
Click on Move.
Choose the project or the sub-project you want to move the file to.
Click on Move to confirm or on Close to cancel.
To move several files:
Select multiple files.
Right click on any of those files.
Click on Move Selected Files.
Choose the project or the sub-project you want to move the files to.
Click on Move to confirm or on Close to cancel.