1. First, if you are going from 8.X or lower TO 9.X, you need to install the following file from Cisco: 'ciscocm.cer_refresh_upgrade_v1.1.cop.sgn'. Do the install from the CER OS Administration page.
2. After this COP file is installed successfully, then you can do the upgrade to 9.X with this file: 'UCSInstall_CER_126.96.36.19900-1.sgn.iso' Again, on the CER OS Administration page.
NOTE** The file named 'Bootable_UCSInstall_CER_188.8.131.5200-1.sgn' is NOT the upgrade file. This file is for new installs only.
Once you do these two steps, you should be up to the 9.X version without issue. Keep in mind, it does take time to do this upgrade. When the server reboots going into the upgrade, expect it to take time.
Some screenshots of what I saw: