After you've selected a 'Best Answer', you'll still see 'Mark as Best Answer' alongside all of the other posts on the thread (except your own). If you simply click on 'Mark as Best Answer' elsewhere, it will switch 'Best Answer' from your initial choice to your new one.
You can see how many BAs a member has got by going to their profile. (Simply click on their username or avatar). For example, the mighty Dannyk13 (one of AB's crossword experts) is way ahead of anyone else
https://www.theanswerbank.co.uk/members/dannyk13/
BAs don't really mean much but those of who've got a competitive streak find that it helps to drive us on to answering more questions. As Bathsheba states, I've got a few more than she has, so feel free to switch your 'Best Answer' to her. I promise not to sulk. Well, not too much, anyway ;-)