You can move a piece in two ways. Firstly, if you have javascript enabled in your browser you can click on the piece and click on the square you want to go to. This will then fill the correct move text into the quick reply box, you can then post to make a move.
The second method is for if you do not have javascript enabled, to make the move enter into the quick reply box the coordinate of the piece you wish to move and the coordinate of its destination in brackets seperated by a comma. For example (a2,a4) note that the exact syntax is required, letters should be lowercase.