You will not be able to edit any milestone that is funded. However, you can make adjustments by releasing the current milestone and creating a new one after discussing with your freelancer.
It would be great if you could just edit aspects of that milestone. For example, I have a freelancer on a weekly payment and I want to increase it. It seems a bit ridiculous to have to end a milestone and start a new one just so that I can increase his weekly payment. Assuming, of course, that I'm correctly understanding how this works. Or is there an easier way to increase the weekly payment that I'm unaware of?
After reviewing and approving the work and when you are ready to release the milestone payment, you will be presented with the option of editing the payment amount of the milestone. You will be able to increase the milestone payment right before releasing it, and therefore creating a new milestone to increase the payment is not necessary. Alternatively you can issue the freelancer a bonus payment.
Here is an article with more information regarding Paying for Fixed-Price contracts.
Thanks, Nina. Apparently, I was a bit confused in regards to Upwork's terminology and it's not actually a milestone that I'm referring to. I've heard back from Upwork's customer support and things are becoming clearer. Thanks for the response!
Dillon, a weekly payment on an hourly contract is not the same as a milestone on a fixed-price contract. It's not really clear from your post what sort of contract you have set up?