If you want to change it to a Knight, mark the checkbox in front of "Move Piece"

(Pawn Promotion to Knight)

Roules for 'Pawn Promotion'

Your Pawn must have reached your opponents ???.

Your Pawn changes to a optional piece, usually to a Queen or a Knight.

You can not change to a King and to keep the Pawn as it is, is just foolish.