According to the standard terms of every Upwork contract, releasing the escrow or approving the freelancer's work diary is an indication that the work they submitted was acceptable to you. While I believe you can request a refund, it has to be within 30 days of the payment. Even then, it would likely be an up-hill battle, as the freelancer would probably decline the refund, and then the escalation steps begin. And since the time for leaving feedback has passed, the freelancer really has no incentive whatsoever to agree to a refund.
This is why it's vital for clients to review a freelancer's work in a timely manner. For fixed price, this means you have 14 days from the date the work was submitted to request changes or to release payment. Again, releasing payment or taking no action in those 14 days, according to the way the system works, is your direct approval of the work submitted.
If it's been more than 30 days since the payment was made, you're pretty much out of luck. If it's been less, you're probably still out of luck. Honestly, I would use this as a learning experience, and make sure to review submitted work in a timely manner going forward. I don't mean for that to sound harsh, but that's really the best way I can put it.