Call the airline.

It will be helpful if you have either the 6 character itinery number (PNR) or the 16 digit ticket number. One or the other or both should be on your paperwork.

Explain what you're trying to do.

Hope for the best, but expect the worst as most tickets bought by non-business types are the very inexpensive non-refundable variety. Most likely you're looking at either buying a whole new ticket or paying a hefty change fee (depending on where you're going, the change fee can easily be more expensive than just buying a new ticket.)

You might be able to salvage something by using your current ticket for the outbound flight and then buying a new ticket to get home - just throw away the return potion of the initial ticket.