Use a pairing knife to slice just around the top of the flan where it may have baked to the glass ramekins. Then, insert the knife down the side and separate a bit of custard from the glass. You don’t need to cut all the way around. It should come loose with just a little bit of separating on the side. You can then take a plate and place it on top of the flan and flip, or you can just quickly flip your ramekin onto a plate and the flan should gently fall down.