The cPanel update failed with the following error :
Fatal: Your RPM database appears unstable. It is not possible at the moment to install a simple RPM.

The RPM database is corrupted which is causing the problem.
Follow the below steps to fix this issue:

1. mkdir /root/old_rpm_dbs/
2. mv /var/lib/rpm/__db* /root/old_rpm_dbs/
3. rpm rebuilddb
4. Then run, #upcp –force

If you require help, contact SupportPRO Server Admin