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.

The Service Pack installation documents I used in the past go into some detail about how to backup, and other measures to reduce risk. Read them in depth.

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) go into some detail about how to backup, and other measures to reduce risk. Read them in depth.