Please refer below document to uninstall FixPack 126.96.36.199 and move back to previous fixpack 188.8.131.52.
"How To Uninstall Siebel Full Installation, Siebel Fix Packs, or Quick Fixes from 184.108.40.206 onwards, 7.7, 7.8 and 8.0 release (Doc ID 477767.1)" on My Oracle Support.
To invoke the Uninstaller in the Microsoft Windows platform, go to the SIEBEL_HOME\_uninst directory and start uninstaller.exe. To invoke the Uninstaller in the UNIX platform, go to SIEBEL_ROOT/_uninst and start uninstaller.
When the uninstaller is launched, you will be presented with two options:
Maintenance Release Uninstall
> Select 'Maintenance Release Uninstall'
> select fixpack 220.127.116.11 to rollback
> follow the instructions and once done you can check 'base.txt', which will show the latest version.
Please mark it correct and helpful if it helps.
Thanks & Regards,