here the answer from Oppo Support

Thank you for contacting OPPO!
We understand that you are interested in the ColorOS update for your Find X5.
In order to proceed with the update to color OS 13.1 please follow the steps below:
1) Please, go to Settings Open App Section > App Management > Find Software Update Application and Allow and Give All Notifications Permissions.
2) Please, check if there are any available updates. To do that please access:
[Settings] > [Software Update]. If there are new updates available, please connect to the internet and allow the phone to update to the latest software.
3) Please, clear update cache. To do that access:
Settings >Apps > App Management > Tap on the 2 dots on the top right > Show system > Select the Software Update app > Storage Usage > Clear Cache