PowerPivot for Excel 2013
Bradley Schacht is a Principal Program Manager on the Microsoft Fabric product team based in Saint Augustine, FL. Bradley is a former consultant, trainer, and has coauthored 6 SQL Server and Power BI books, most recently Learn Microsoft Fabric. As a member of the Microsoft Fabric product team, Bradley works directly with customers to solve some of their most complex data problems and helps shape the future of Microsoft Fabric. Bradley gives back to the community through speaking at events such as the SQLBits, Fabric Community Conference, PASS Community Data Summit, SQL Saturdays, Code Camps, and user groups across the country including locally at the Jacksonville SQL Server User Group (JSSUG). He is a contributor on SQLServerCentral.com and blogs on his personal site, BradleySchacht.com.
Then select Add-ons 
Next select COMM Add-ins from the Manage box and click Go 
Check the box next to Microsoft Office PowerPivot for Excel 2013 and click OK (Optionally check the Power View box while you are here as well)
At this point you are good to go with PowerPivot 2013! I have the PowerPivot 2012 RTM on my machine so it looks like Excel automatically picked that up as being enabled. In my case I had to uncheck the PowerPivot for Excel box (which was the 2012 version) and then check the box for Microsoft Office PowerPivot for Excel 2013 in order to get the newer version. If you don't have a previous version of PowerPivot installed though you don't need to worry about unchecking the PowerPivot for Excel option.