The rebooking- and cancellation insurance can be found in the "Additional service" step in the booking. With this, you can rebook the date or time, or get your money back if you choose to cancel the trip before departure time. A trip can be rebooked once and canceled once thereafter.
The rebooking and cancellation insurance is not refundable and you can not add it afterwards. If you have not chosen the rebooking- and cancellation insurance your trip cannot be refunded, but you can rebook on ybuss.se for a surcharge of 100 SEK.
You can manage you rebooking or cancellation here