My 2014 yeti Bolero radio reception was, rubbish station fading in and out with static, so decided to change the head unit for a Xtrons with Android unit thought this might cure the FM radio reception problem but it didn't , so now thinking it might be the aerial how easy are they to replace I'm assuming the flexible rubber bit just screws off and there must be a screw holding the base onto the roof, any advice gratefully received.

Your first move should have been to check that all the aerial connections were clean and proper.
Even just unscrewing the twig, cleaning and refitting it  might improve things.

In some cases the heated rear window is part of the aerial arrangement. I had to have my rear window replaced and the reception was truly awful - prior this it had been fine. It turned out that there are alternative rear windows, one with two tabs for the electrics, the other three. The third tab relates to the aerial. The rear screen was replaced with the three tab option and problem solved.