It's very easy !
1. You download the latest firmware
2. You put your phone in fastboot mode
3. You connect your phone with the original cable to an available port on your motherboard
4. You open the "OppoRealme-OFP-Flash_1.0.exe"
5. You choose the "most logical" options
The files will install one by one and you may get a lot of errors!
Leave it, the program will continue and at some point there will be a "super" file which will take a long time to install (30 minutes), you wait and then you will see your phone restart as if it were brand new !