As a best practice it is recommended that backups of the database, assets, and index be scripted and maintained on a daily basis. See the documentation for downloading/restoring indexes from the command line interface.
Without a matching index, it is possible to restore a dotCMS instance using only the database and the assets files. However, after this type of restore, it’s imperative to run a full re-index.
If you have a dotCMS zipped backup you may upload it via the Maintenance task or place it under /dotCMS/starter.zip and start dotCMS. Keep in mind that a restore will delete your current database and place it with the one you are restoring from.