This is my first entry in the Fantasy Premier League

I have no idea how I managed to get my goalkeeper (Alisson Becker) to be vice-captain and a striker (Joao Pedro) to be be captain. I did not realise until now that I could select them and earn extra points from them.
Well, I tried but cannot work out how to change the captain and vice-captains. Maybe I am missing the obvious but nevertheless, can they be changed?