Backup all your Config and Content databases, and your servers, before doing the update. If your servers are VMs, snapshot them before the update. You'll need to have a maintenance window to do the update. Depending on the size of your content, you may need a weekend.

You then have a restore point you can go back to.

Microsoft's Service Pack installation documents (find them on the [Sharepoint Products and Technologies Update Resource Centre][1]) go into some detail about how to backup, and other measures to reduce risk. Read them in depth.


 [1]: http://technet.microsoft.com/en-us/office/sharepointserver/bb735839