The cash out option is usually offered as part of 'in-play' or 'live betting'. This is betting while the event is actually taking place, for example, placing a bet on a horserace while the race is being run or a football match that has already kicked off.
After the bet is live, the option to cash out will appear on this screen if it's available. You might see an offer right away after the game starts, but it could also take some time for one to appear. Additionally, there's no guarantee that the option will be there for every second of the contest.