You would need to drain down at least the boiler - I am not familiar with the Baxi, but assume it would have an isolation valve on the flow and return pipework, and a drain-cock somewhere on the boiler.  You would then be able to change the PRV valve.

BUT...

Is this PRV valve 'leaking' because it has gone duff (some crap stuck to the valves seat, causing it to let by), or is it doing it's job?  Is the pressure in the system normal, or gone to high?  If it's gone too high, it could be due to a failed expansion vessel, or because mains water pressure it's finding it's way into the system via a dodgy filling loop, or a failed heat exchanger (combi DHW).

Scruff will help later...