That is not really how this works.
If you would like to ask the freelancer to issue a refund, she is allowed to do so.
But there is no button for you to click, nor is there a procedure you can go through, to get a refund for the money you have already paid to the freelancer.
If you are not pleased by the freelancer's ability to keep to your schedule, then the appropriate action is to close the contract and stop working with this freelancer.
If this is a fixed-price contract, then you may discuss the matter with the freelancer and come to an agreement about how much she should be paid for her work thus far, out of the money remaining in funded milestone escrow. She can even agree to get paid zero.
Then you can close the contract and edit the amount of money to be released. You can put in whatever amount the freelancer agrees to. She will be asked to approve the edited amount. And you will get a refund.
But if time and deadlines are an issue, the fastest way to deal with this is to not talk to the freelancer at all. Just close her contract, release all the remaining money to her, and be done with it. If the freelancer has not respected your deadlines, you do not owe the time or respect required to negotiate lower payment with her in order to set her karma right.