This can allow you to make a profit early or limit loss.
To sell the shares you've previously bought you go through the same process as when you first bought the shares - only this time you sell rather than buy. The system will always calculate a net position in the same market, so if you buy 25 shares and then sell 25 shares you're left with no shares and no stake in outcome of the underlying event (you've completely exited your prediction).
If you're selling your shares click the red sell button, enter the number of shares and the price you want to sell at and confirm your order. Please be aware that if you sell your shares for less than you bought them for you'll make a loss. If you sell for higher however then you'll make a profit.