Thank you for sharing your question about removing a screw from your buffet table.
It is an old flathead screw. The straight slot on its top gives it away as a type of screw. I suggest using a Craftright 19 Piece Screwdriver Set specifically the 1mm flat head screwdriver. The set does have a smaller piece if the one I recommended does not work. If the screw refuses to lift out your next step is to use a Trojan 250mm, Nail Puller.
It sounds like an interesting upcycle project. Any updates you can provide would be much appreciated. Are you painting it a new colour or are you revarnishing the table? If you have any other questions about your project please don't hesitate to post them. Our members would be more than happy to provide advice and information.