Order of Installing Service Pack on systems hosting Database Mirroring

If you have servers which hosts database mirroring and you want to install the service pack on those systems and you are trying to find out the order in which you need to do it then, here is the order in which you need to install service pack or a hotfix on the systems which hosts Database Mirroring.

1. Backup the principal
2. If you are using a witness, remove it from mirroring
3. Upgrade the mirror
4. Failover to mirror
5. Upgrade original principal/current mirror
6. If you wish to fail back to original principal continue on; otherwise, proceed to step 8
7. Failover to original principal
8. If you are using a witness, add it back into mirroring

For more information refer to the following article to know the entire process for upgrading both the principal and mirror servers.
http://technet.microsoft.com/en-us/library/bb497962(SQL.90).aspx