I have a fully executed contract and wish to change the name of the buyer on the contract

You cant. It would take a new contract. Why do you want to change the name?

If you're the buyer then you can request that the seller allow you to assign the contract to the new buyer. If you're the buyer and you want to add another buyer then you can request that the seller allow an amendment to the contract.

If you're the seller then you would have to cancel the contract and you would need the current buyer's agreement to do so.