Welcome to the forum Gary. The part in the centre will pull out (sometimes with a twisting motion), but the outer ring is held in position from underneath the shower tray and is sealed to stop water leaking out. Google shower waste.
if its been put together properly you should be able to unscrew the waste fitting and replace with similar but if its been covered in silicon or the trap drops too far to reach your in trouble
I unscrew these by wedging the head of the grips inside
Looks like a mcalpine shower waste, google "McAlpine STW3-R"
you get a key with it that fits in to tighten and loosen that outer top part, carefully if you have not got access underneath it though as you might undo it and the bottom waste part fall away out of reach.