Help - Can I add a milestone to an existing contract?

[leahrendon]

I completed some work for a client a few weeks back. They came back and asked that I do some additional work. About two hours worth. Both the client and myself are very new to Upwork. I told them I couldn't hand over the new work until they issued a new contract or added a new milestone to the existing contract. I wanted to make sure I was covered and I'd be paid for the work before I handed it over. But they haven't responded in DAYS. They're incredibly slow to respond so I'm not worried but the due date came and went and I didn't hand in the work because they never created a contract or milestone. Would I be able to add a milestone to the existing contract on my end and turn the work in through Upwork, or should I wait for them to do it. Sorry. This is all new to me! Thanks for any help anyone is able to offer.