Should you wish to Cash Out, all you need to do is click on the Cash Out button located below your selection(s). If you choose to Cash Out your entire bet, this amount will be settled and the final result relating to your bet will have no impact on the amount returned to your account.
However, winning too often will see your account with bet365 restricted. This is why bookmakers don't like professional gamblers who consistently win. It is bad for business, and bookmakers are out to make money, not lose them.