Upgrading Instances with Mirroring If you want to upgrade or install a patch on two separate instances that house both the principal and mirrored database, how can you go about that?  If the database that is being mirrored needs to be available 24/7 and you don't have a window to go offline, what is the best means to do this?

EDIT: this is with SQL Server.
First hit from Google gives an MS KB article

"How to install service packs and hotfixes on an instance of SQL Server 2005 that is configured to use database mirroring"

Another viewpoint "How To Apply Service Packs To SQL Servers Involved In Mirroring"