Puzzle pieces can only be changed during the creation of the puzzle. Tap the option for your desired size, or use the right or left arrows to scroll for more options.

Max Pieces are as follows

iPhones | 225 pieces

iPhone 6+ & 7+ | 400 pieces

iPad | 1024 pieces

If you have started a puzzle and would like to change the amount you will need to return to the home screen and select the puzzle again.