Experienced travellers will know that one way tickets to the US are very expensive compared to the price of round trip tickets. As you have found out this should have been booked as a round trip ticket from the outset rather than 2 separate one way tickets; but that it is too late now. You can't rebook /change the ticket to a round trip ticket - also because you will incur high change fees by doing so.

All you can do now is to hope that you have a significant schedule change or cancellation before departure which will allow you to cancel the ticket and get a full refund. Then you can book a new round trip ticket.

Sorry to be the bearer of the disappointing news.