1. Take backup of all dbs in Principal Server
2. Pause Mirroring on Principal Server for all mirrored dbs
3. The Principal server databases becomes (Principal,Suspended)
3. Stop all SQL services on Mirror Server (Including WMI service)
4. Apply CU patch on mirrored servers
5. Unpause Mirrors (wait for them to catch up)
6. Failover the databases (the Mirror server becomes Principal server)
7. Pause Mirroring
8. Hand-over to Application users to test and wait for their confirmation to proceed.
9. Stop all SQL services on Mirror Server (Including WMI service)
10. Apply CU patch on Principal servers
11. Unpause mirrrors
12. Failback databases.