I’ve been out of the loop with oneplus devices for a long time so can’t be much help.
That XDA link is the place to go for starters. Checkout post #300 which broadly confirms my thoughts, i.e: you need specific firmware package for downgrading from major android version to another.
I remember doing this from 12 to 11 on device, and I think a wipe of data is involved. In your case you would probably try the C.62 downgrade zip for whichever variant of phone you have, global, eu or india.
The basic steps to do this are described in my old post that @piero has linked.
Is the local updater available in the settings under oos13 by the way? If not you may need to install something.
MSM download tool is another option but you need windows pc/laptop for that. and some good instructions from somewhere on the forum. I’ll have a look to see if I can find my old links.
Maybe waiting for eOS rollout of android 13/T would be easier!