Usually the Mirroring feature of SQL Server is pretty resilient to random failures at several parts of the link between the two servers. It is so resilient because it uses only built-in features that are not affected by the outside world – so you can boot your servers, cut and reconect the link, do a lot of stuff and the database will still resume mirroring once everything is good again.
But a Service Pack is something that messes with those built-in SQL features, so this can definetily mess up with your mirroring session. Please be safe and perform these kinds of upgrades gracefully. Kinda, like reading this link: http://www.mssqltips.com/sqlservertip/2701/steps-to-apply-a-service-pack-or-patch-to-mirrored-sql-server-databases/ .
Because if you fail to do so, you may end with two suspended databases. And in this case you will have to take all your servers down, upgrade them all to the same SP level, restart them all, make use of the three little commands noted below, and pray for the database god/goddess/entity to resume your mirroring session. If everything goes fine, you will see your mirroring session back to “Synchonized” state.
See, I can not guarantee that this will work. But, you know, mirroring is resilient.
USE MASTER GO ALTER DATABASE <your_db> SET SAFETY FULL GO ALTER DATABASE <your_db> SET PARTNER RESUME GO --optional ALTER DATABASE <your_db> SET SAFETY OFF GO