Although the answer "get back on any way that you can" is on the tip of our tongues, we will share with you the right technique for getting back on your board.
First, get your paddle out of the way by placing it on the front of your SUP so that you have both hands free. Next, get yourself to the side of your board, level with the carry handle and grab the opposite side or the carry handle with one of your hands. Finally, haul your body onto the board, holding your board with the other hand. It doesn't matter what it looks like, you're back on your board and you can continue on your adventure!
A piece of advice: if you are right-handed, it will be easier for you to get back on from the left side, whereas if you are left-handed, the opposite would be more comfortable.