Download the SP1 package from this location
http://www.microsoft.com/en-us/download/details.aspx?id=42008
As with any CU you should backup and update your servers that are running SharePoint.
1. Install the SP1 package by running the package executable.
2. Finish the install by running PSCONFIG.
3. Ensure that you farm has been undated and all service applications are operational.
Here is a great link from Stefan Goßner for a FAQ on the SP1 release for SharePoint 2013
http://blogs.technet.com/b/stefan_gossner/archive/2014/02/27/sp2013-sp1-faq.aspx
In addition to the questions that are answered in the FAQ above, some additional topics may be of interest:
What about our workflow servers?
- no update required if you have already installed the update for workflow as described in my March CU article
http://blogs.msdn.com/b/chhopkin/archive/2013/04/04/sharepoint-2013-updates-as-of-april-2013.aspx
What about web apps server?
- no update required if you have already installed the update for Office web apps as described in my March CU article
http://blogs.msdn.com/b/chhopkin/archive/2013/04/04/sharepoint-2013-updates-as-of-april-2013.aspx