The Backup Project tool creates archives of projects. Each archive is a single file with a .PRJ extension that contains the project's folder and all of its contents along with related data from the SQL project database.
Select Project Manager > File > Backup
Or, right-click the name of the project in Project Manager and select Backup from the context menu
The backup file will be created in the path specified by the Save in field on the Backup Project dialog. The file will have the project name as the file name and will have a .PRJ file extension. You may need to enable the display of file extensions to see the .PRJ extension.