In order to change the date on the Oppo-Smartphone, the automatic time must be deactivated. Therefore it is also necessary to adjust the time if necessary. A separate setting is not possible under ColorOS.

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  Date
  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 Date

Oppo Instructions