It may not exist, in some BIOS' it does not and is hidden/enabled by default..If it does exist, it would most likely be in the BIOS Advanced Settings options in the Chipset/PCH settings.
 
If it does not exist, you can run Command Prompt as administrator..Then copy/paste in the following command line, then press Enter (and confirmation should be displayed), close Command Prompt then reboot to disable HPET in the OS: bcdedit /deletevalue useplatformclock
 
To enable HPET, use: bcdedit /set useplatformclock true
 
Even after disabling it in the BIOS, if the option exists, you should still disable it in the OS..Disabling it in the OS would override the BIOS setting anyway.
 
To confirm that it remains disabled after reboot, run bcdedit /deletevalue useplatformclock again and if you get the "An error occurred" blah blah "Element not found" message, then it's disabled.
 
There's tons of articles about it if you search..Like here.

Thanks for your instructions, now I've disabled it in the OS, Wintimertester shows 3.2blabla MHZ. No difference observed, perhaps becuz it's already smooth. btw I play Overwatch.