The SO status became overdue since it is passed delivery date and the billing status is not 100% after creating Sales Return.
Scenario:
Is there a way to change the SO status from overdue to “To Bill” or “Completed”?
You could change the status to “Closed” using the Status button. I think that’s really the only option at that point, as a control against fraud, etc. If you ever needed to, you could re-open the Sales Order after it is closed by the same method.
But why does SI Return affect billing status of SO? Should the SO stayed Completed after creating Sales Return? I was not able to close SO 
Well, like you I’m just a user and I’m speculating as to the reasons for this. But what I’ve observed is that the sales process is very tightly connected from Quote-Sales Order-Sales Invoice-Payment. It seems to be made to progress with all the same numbers. If any of them no longer matches, even in the future, due to a return, then it’s designed to show the sales process is not complete. I believe this is probably a feature for management control of the sales process to prevent “funny business” where people get commissions for unrealized/fake revenue.