The documentation is quite clear about this. Excerpt taken from http://ravendb.net/docs/server/administration/backup-restore :
Restoring a database is an offline operation, it cannot operate on a running instance of RavenDB.
Raven.Server.exe -src [backup location] -dest [restore location] -restore
If the restore location doesn't exists, RavenDB will create it.
You cannot restore to an existing database data directory, the restore operation will fail if it detects that the restore operation will overwrite existing data. If you need to restore to an existing database data directory, shutdown the database instance and delete the data directory.
So if you have a database "Test" that you have a backup from, and you want to restore a backup for that database, then you will have to delete (or rename) the data directory of that database ("Test" in this case). RavenDb may not be running (= offline) for the restore.