How do I upload a complete folder to an existing project?
The easiest way to upload a complete folder to an existing project on writeLaTeX is to:
1) Create a new empty folder in Overleaf, via the projects menu.
2) Click on the arrow to the right of the folder name, and select "Upload from: Computer"
3) 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).
4) 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.