In general, you can cancel a tour and receive a full refund up to 14 days before the tour departure, with some exceptions that are indicated in the tour description. To cancel a tour, you can call the number on your excursion confirmation during regular business hours.
However, if you cancel within the 14-day window before the tour departure, most tours become non-refundable. This is because the tour operators have reserved space for you, and it may be difficult to fill that space with another customer so close to departure.