If it's anything like my Triton (x300si IIRC) then it's pretty easy.
My surprise was that it was such a crude arrangement in the first
place ;-/.

You unscrew the cover screws, and then undo the nut holding the exit
pipe arrangement onto the boiler output. There are a couple more
screws to undo to get a fitting out of the way, and then the PRD can
be pulled out. Refit is the reverse. You need to be careful to ensure
that the o-ring fits properly, and that everything is snug so that
tightening up the screws causes it all to lock in neatly. Given the
design I was amazed it didn't leak in the first place.

I have also considered fastening the overflow pipe onto the PRD in
some way - it's a pretty loose fit and I have have water coming
through the ceiling below when it came off and the PRD ruptured.

Nowadays I keep a spare PRD handy as I've had them go a few times.
Teenagers having too many showers, shower head not being cleaned, that
kind of thing...

HTH
Jon N