Hi,
Apologies for the inconvenience.
I’m afraid that it is not possible to select a single sub-site to restore from an external database backup in the same manner as restoring an UpdraftPlus backup.
The UpdraftPlus multi-site backup includes additional metadata which documents the site structure in the database backup.
However, you can adjust the external backup to only include the sub-site you wish to restore. If the database backup file only includes the tables for the sub-site, then restoring the backup will only affect the sub-site (i.e. UpdraftPlus will only restore what is contained in the backup file/database dump)
Best Wishes,
Dee