If you have purchased your ticket online, it can be rebooked, depending on the type of ticket you have. If you need to rebook the ticket to a more expensive departure time, you will have to pay the difference between this and your previous ticket. 
Take a look at the ticket rules here

Call Alsie Express on +45 88 81 11 11, if you want to rebook your ticket.

Note: the price of the individual departures may vary.

If, on the contrary, you wish to cancel your ticket entirely within 24 hours irrespective of type of ticket, this is done free of charge and with a full refund of the amount.

Transfer of ticket

If you wish to transfer your ticket to another person, this can be done up to 24 hours before departure.

If so, please contact Alsie Express on +45 88 81 11 11 stating that you will not be using the ticket yourself and provide name and of the person to whom you wish the ticket to be transferred. Please note that an adult will not be able to take over a child's ticket. (unless you pay the difference?)

Book your ticket online