If you haven't yet booked a ticket, then you have two choices:

1. Book a flexible ticket that allows you to change the return date, and initially book the return journey for whatever date you guess you'll want to return.

2. Book two one-way tickets. Be warned that this may be quite expensive.

If you've already booked your ticket, you can probably (but not definitely) make changes to the return journey for an extra charge plus any difference in fare. The fare rules should specify whether this is possible.