Seller decides he doesn't want to uphold his part of the contract

[Cole Godshall]

I am currently under contract on a house where there are some standing issues with the house. The seller agreed to replace all the windows in the house since they are old and outdated for an increased sale price. We both signed this contract back on 11/5 with a closing date of today, 12/5. All of a sudden yesterday, 12/4, I got an email from my realtor stating that the seller has not done the windows yet and apparently is no longer is willing to do them. The seller is trying to re-negotiate the contract to not include the windows but I don't want to do that because I don't have the money to pay for those out of pocket. The cost of the windows is roughly $15-20k and the most I can get back from seller credits is $11.5k. Any tips on what I should do about this?