Flight question

[cdcox]

I needed to make an emergency trip with uncertain return date. Rather than buying two one-way tickets, I bought a round trip ticket with my best guess at a return trip. My return flight is Thursday but I'm not ready to go back then and I'm still don't know my exact return date. How do I cancel my return flight while still holding the ticket open? I know I will pay a change fee. Do I need to call Delta or is there a way to do it online?