Here are 3 handy resources to have a read through before going ahead with the upgrade.
Well worth a read through the full section for the SP2 upgrade as it applies to R2 Sp1 too. So the check list, considerations and planning to upgrade to Sp1 right at the end of the article.
Expand the “To test….” section for better instructions on how.
And finally, a great Step-by-Step SCCM 2012 R2 Sp1 Upgrade Guide in pictures.