You can cancel a bet at any time if the odds have not changed.
You just need to click on "My Bets", in the upper right corner, and click on the bet you want to cancel and then on "Cash out". If there are no changes in the odds, the full stake will be refunded.
If quotes have changed, the "Cash out" option will still be available, but the refundable amount will vary as quotes change.
Whenever a pre-match bet is placed and Rivalo does not offer in-play bets for the event, you can cancel bets on the event until the start of the game, if in-play bets on the event are available, you can still cancel full refund bets if the odds have not changed.