If you have the Autodesk Application Manager installed hopefully you have seen these two updates.
Revit 2015 Update Release 1 was released previously.
Revit 2015 Update Release 2 is referred to as the Security Update. This security update includes all Update Release 1 improvements. Additionally it addresses Heartbleed vulnerability (a security bug in the open-source OpenSSL cryptography library).
Here are the direct download links:
These links can also be found on the Revit Products Download page:
After installing the Security Update the Revit 2015 build should be:
Update Release 2