Download UR8 for Revit 2015:
For R2
http://download.autodesk.com/SWDLDDLM/Updates/Revit/UR8/Autodesk_Revit_2015-x64_Update8_forR2.exe   

Not for R2
http://download.autodesk.com/SWDLDDLM/Updates/Revit/UR8/Autodesk_Revit_2015-x64_Update8.exe

Enhancements List (R2):
http://revit.downloads.autodesk.com/download/2015RVT_RTM/UR8/Enhancements_List_RVT_2015_UR8_forR2.pdf

Readme (R2):
http://download.autodesk.com/SWDLDDLM/Updates/Revit/UR8/Autodesk_Revit_2015_R2_UR8_Readme.htm

Not for R2 – Readme
 http://download.autodesk.com/SWDLDDLM/Updates/Revit/UR8/Autodesk_Revit_2015_UR8_Readme.htm

Not for R2 – Enhancements
http://revit.downloads.autodesk.com/download/2015RVT_RTM/UR8/Enhancements_List_RVT_2015_UR8.pdf