Yes you can! You can submit a reschedule request even after the departure date has passed, as long as it is still within the ticket validity period. Different airlines have different ticket validity periods.

For your information, each airline also has different policies regarding fee for rescheduling flexible tickets. You might have to pay a fee, but be sure that the fee will still be cheaper than the fee imposed for rescheduling a regular ticket from the same airline.

You can see the policy details on your e-ticket by tapping Reschedule and then Read Policy.