I guess you will have a little research to do, perhaps start here https://community.oneplus.com/home … this Edited is the caution from the Upgrade link
Caution: Before following these instructions please ensure that your smartphone is on the latest Android firmware available for your device. For example if available, before flashing /e/OS ‘S’ install Stock Android 12.
Check you device vendor’s website to download the required Stock ROM.
The point being that you need latest, I am not certain what that is for fajita, sorry.
I did find this, but please verify for yourself as well ! https://service.oneplus.com/uk/search/search-detail?id=2096330&articleIndex=1
So I will lose all my data?
Yes one or more backups should be taken before you start.
As the jump from Android 9 to 12 is so big, I would uninstall all apps that you really do not need a backup of … even the best app backup might not survive such a jump, so keep your backup simple, or essential only, I suggest.