In multi-master replication, the other masters each contain an authoritative copy of the replicated data. You cannot restore an old backup because it might be out of date with the current replica contents. If possible, allow the replication mechanism to bring the master up to date from the contents of the other masters.
If that is not possible, restore a multi-master replica in one of the following ways:
The simplest way is not to restore a backup, but to reinitialize the intended master from one of the other masters. This ensures that the latest data is sent to the intended master and that the data will be ready for replication. See Replica Initialization From LDIF.
For replicas with millions of entries, it can be faster to make a binary copy to restore a more recent backup of one of the other masters. See Initializing a Replicated Suffix by Using Binary Copy.
If you have a backup of your master that is not older than the maximum age of the change log contents on any of the other masters, the backup may be used to restore this master. See To Modify Change Log Settings on a Master Replica for a description of change log age. When the old backup is restored, the other masters will use their change logs to update this master with all modifications that have been processed since the backup was saved.
Regardless of how you restore or reinitialize, the master replica will remain in read-only mode after the initialization. This behavior allows the replica to synchronize with the other masters, after which time you may allow write operations, as described in Restoring a Master in a Multi-Master Scenario.
The advantage of allowing all replicas to converge before allowing write operations on the restored or reinitialized master is that none of the hub or consumer servers will require reinitialization.