You cannot change the board after the post has already been published. If you chose the wrong board, then we advise you to delete the post as soon as possible and make a new post on the correct board. The sooner you do this, the less votes and comments you will lose.
In some cases, platform administrators themselves can change the board for your post if there is a more suitable board for the post, or if the post violates the rules of the board on which it was originally published.