E-Filed Return And Was Accepted - Need Change "Can Be Claimed As Dependent"

Hi,

 

About half a month ago I e-filed my father's return as if he cannot be claimed as dependent on someone else return. The IRS already accepted it, and his return had 0 balance (no refund & no payment due). How can I change the filing in order to claim him as a dependent in my taxes?

 

Thank you for your help

 

Nick