Selling your position is simply putting it back out to the marketplace at a new price. So for example, if you have $10 on Vikings to win at 50% probability or -100 (pays out $20) and they go up by 21, you can decide to sell that position at a new more relevant price (i.e., since it is more likely to occur). If someone decides to buy it, then you will be able to take some winnings and offload the risk to the buyer.
Likewise, if the Vikings go down 21, you can try to sell it for a lower probability (price) to cut your loss and offload the position to a buyer.
In both scenarios, you will need to price it relevant to how the game is going for another user to actually buy it. If no one buys it, it just goes back to your position at the expiration date time.