Upload Folder:

When creating or updating an existing folder, there is an option to make the folder an Upload Folder. This gives tenants or other non-Document Library Admin users the ability to upload files into the specified folder. 

Once the "Upload Folder" checkbox is selected, all users granted permission to the folder (See Permissions page for more information) will be able to upload files to that folder via the My Workspace page.

Click "Upload" next to the folder name. The File Upload screen will appear. Select the desired file by clicking "Browse" then click "Upload" at the bottom.