Trying to shift both U and D from my board but it has to be done dynamically

.U...UXU..
SUDXDUXUDT
..DXD...D.

That's my board for the game with dimensions 3 10

Here's my problem. I am unable to write a code that shifts all the U one unit up and all the D one unit down without double shifting. And if the U or D are in the end of the board shifting them down or up will result in them emerging on the top and bottom edge of the board but within the same column.

Can you help please?