If you want to change the time manually, you must deactivate the "Use time from network" function on the Oppo Smartphone, after which the menu item appears so that you can change the time.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  Additional settings
Step 3:  Tap on  Date & time
Step 4:  Disable  Network-provided time
Step 5:  Tap on  Set time
Step 6:  Adjust the  time
  1. Open the Settings
  2. Tap on Additional settings
  3. Tap on Date & time
  4. Disable Network-provided time
  5. Tap on Set time
  6. Adjust the time

Oppo Instructions