How can I add a manual tag to an upload?

Click on an upload to open the clip preview modal (on the Uploads or Library pages), in the top right hand corner you will see the ability to add tags to your content

How can I add a manual tag to an edit?

On the Edits page on the Edit sidebar, you can add manual tags. You can also add tags to an Edit from the Library when you click on an Edit to open the clip preview modal