The easiest way to upload a complete folder to an existing project is to:
- Create a new empty folder in Overleaf, via the projects menu.
- Click on the arrow to the right of the folder name, and select "Upload from: Computer"
- Navigate to the folder whose contents you wish to upload, and select all the files (e.g. by holding down shift and clicking on the first and then the last).
- Click "Open", and your files will be uploaded to the folder you created.
And you're done!
PS: To upload a complete project simply go to your Projects dashboard and click the "Upload zip" button on the right sidebar.