Upvote:5
Depends a bit on the airline policy and the specific fare class and ticket rules.
In most cases, the airlines will simply cancel your return ticket as soon as you miss any leg of your itinerary. If you want to salvage your return flight you need to execute a change: you need to change the return ticket to a one-way ticket. You will have to pay a change fee (depending on your fare class and ticket conditions) and the difference in price between your original ticket and the new one-way ticket. In the unlikely event that the new ticket is cheaper, you can credit the difference against the change fee.
In most cases with this will be quite expensive and it's good practice to shop for alternative tickets so you know what your options are before you call.
Some "basic" or "light" tickets cannot be changed and you'll loose the entire ticket if you don't fly it exactly as booked.