You'll need to replace the board. Over time, anything you use to seal the wood will break down and you'll have the problem all over again. Buy the board but don't install OR cut it right away. Leave it lay on the deck for a while to dry out and shrink. Flip it every few days. When it's dry and all, remove the old board and cut the new one using the old board as your template. If the old board was nailed, buy some 3-1/2" deck screws. Pre-drill the new board so you don't split it when you screw.
In the meantime, you may be able to flip the board over and use the other side. This depends on the board on the other side. If you damage the 'top side' when trying to remove it, don't worry because you'll be flipping it over. Just try to minimize the damage you do to get it out (especially if it's nailed in place.) Buy a 'cats paw' if it's nailed in.
My best to ya and hope this helps.
PS: You may be tempeted to just use the bottom side on a permanent basis. You can try this if you want. Clean the bottom side, let it dry, and install it face-up. At worst, later on, if this doesn't work out, you buy the new board and do as I described.