Hi Karim,
If you are restoring to the same site it was created on, then no direct action should be needed – the backup should shop in the console of the site, and UD will automatically download it.
If you are wanting to restore to a different site (i.e. a migrate operation), then you currently have to download from Dropbox and upload into the new site. A future release of UD will have the feature to re-scan remote storage and find any backups there that other sites created – but that’s not yet possible. (Can’t indicate the timescale yet, either).
Best wishes,
David