Water pressure reduction valve adjustment?

[Scarletpimpofnel]

I read that my prv should be set at 3.1-3.8 bar. How do I set the prv below?

Is it as simple as turning the knob? I read you need to turn off the water and release pressure before adjusting? Is that right? Also there may be a locknut but I can’t see one? Finally is there a filter chamber that needs flushing, if so how? Many thanks.

[Scarletpimpofnel]

Thanks all.

@Jagnet ... I have taken your sage advice!