It is generally a very safe operation, but if it fails, it will do a very good job of rolling it back to where you started all by itself.
Once a service pack is installed, if you decide to uninstall it, you would just do that in "Programs and Features", the way you would uninstall any update.
That said, to guarantee successfully (un)install, Microsoft has developed the "System Update Readiness Tool". If you want to improve your odds that it will install without error, run this tool. It can take anywhere from 15 minutes to 2 or more hours to complete, but it will fix underlying problems that you will never find on your own.
As always, to be sure, you should have an image of the computer.