how do i extend the job due date?

[#1]

Today is the due date/closing date and we need about a week for completion. This job has been paid for in full. 

[#2]

As far as I know you can change the due date if the milestone that hasn't been funded just by editing it on the right next to the milestone title, if it has been funded then you can't, but you can just keep the job open until the work is finished, I believe.

[#3]

In general, Upwork doesn't care about milestone due dates. They're just a guide for what the freelancer and client agree is due and when it's due, so original dates don't need to be changed in the Upwork system. As long as both sides agree, all is well from Upwork's point of view.