Requirements:
Download SCCM 2012 R2 RTM bits
Download Windows 8.1 ADK
Steps:
Start upgrade from the top in hierarchy, first CAS then Primary and then Secondary Servers.
Take backup of CAS as well as Primary servers. Ensure that you’ve a good copy (I would suggest to have 2 copies just in case) of backup.
In CAS server, Uninstall Windows 8 ADK from Programs and Features
Install Windows 8.1 ADK along with three features (Deployment Tools, WinPE and USMT)
Disable the site maintenance tasks “Backup Site Server”, “Delete Aged Client Operations” and “Delete Aged Data Discovery” on SCCM ConfigMgr sites.
No need to update remote Site System servers (remote DPs, MPs, SUPs). However, ensure that prerequisite for installed site system roles.
Open the SCCM 2012 R2 bits and run the Splash.hta
On the Getting Started page, make sure that Upgrade this Configuration Manager site is selected.
Follow the GUI and finish the installation.
Once installation is successful in CAS, then follow the same process for Primary server.