I'd like to replace the speakers in the trim behind the seats, for slightly bigger and better. How do I remove those two speakers? I'd like to explore what room there is behind there for upgrades.
You will struggle to fit anything bigger without major surgery but anything will be better than the £1 4" items supplied with the car IMO.
The trims are just lots of screws and a few clips IIRC.
If you have the standard six speaker set up (2 in the footwells, 2 tweeters in the door cards and 2 in the mouldings behind the seats) there is little point in trying increase the 'power' of the rear speakers without a lot of work.
When I changed my interior, I removed the tweeters (as I couldn't use them) and replaced the 2 rear speakers with aftermarket 4" full range Pioneers from Halfrauds (they were not expensive). I needed to replace them as one had blown and was not working anyway.
The car's speaker wires are tiny (like old bell wire), so you will not improve the power, without replacing this, even if your amp can supply the power.
My car now has the original footwell speakers (one of which has the crossover unit attached to it) and two full range Pioneers in place of the original rears, all on the original wiring. My head unit is a cheap JVC with a 50 x 4 watt amp built-in and the sound is OK. It's not 4,000 speaker, modern H&K, or Bose good like in many newer cars, but it is acceptable IMO.