If the OTA is not showing up for you (perhaps you skipped a version and the incremental path is broken), you can manually install the update. This is safe and does not void warranty if done correctly via the official storage.
: Visit the official OPPO Software Update page and find the ROM specifically for the oppo cph1933 ota update
How to Update Your OPPO A5 2020 (CPH1933) to the Latest Software Keeping your OPPO A5 2020 (CPH1933) If the OTA is not showing up for