As I mentioned in a previous post (Don't install SharePoint 2013 SP1 just yet...) Microsoft had taken down the ability to download service pack 1 for SharePoint Server 2013. As of today (4/22) it has been rereleased by Microsoft and is available again to download.
Based on installation instructions in Bill Baer's Blog -
Customers with Service Pack 1 Deployed
For customers who have previously deployed Service Pack 1, download the updated Service Pack 1 and install over the existing Service Pack 1 running PSConfig or PSConfigUI immediately following.
Customers without Service Pack 1
For customers without Service Pack 1 deployed, download the updated Service Pack 1 package and deploy as per the KB documentation.
The KB articles are also different for the for the rereleased service pack. Here is the updated link: KB 2880552 - SharePoint Server 2013 SP1