for tax purposes, how do you change primary property

[HerHonesty]

we want to change our primary residence e.g to another property we own. any idea how you go about doing this formally...

PPR is a matter of fact, so you actually have to make the other property your PPR.
As long as it is your PPR then that should be enough.
You could document this yourself to have the dates.

I don't think you have to formally notify anyone but could demonstrate it by having , say , bank statements, tax forms, driving licence etc altered to that address.

I used to work in Council Tax so for those purposes you have a "sole and main" residence so it's the same sort of thing.

We were looking for other stuff such as schools where your children went, doctors, dentist all being within the area of your "sole" residence or primary in this case.

Council tax billing would also show where your main residence is as you would pay a full charge on one property (providing there are 2 adults living there) and a second home discount on any property you own but don't rent out.