Usually there are only a few plastic buttons on it, which is easy to open. After opening, you can see the long cable that operates the fuel tank cover. Just pull it. If it doesn't work, it will move, moving the movable part of the locking mechanism and pulling the cable at the same time. Some cars will install a special locking switch on the locking mechanism.
In order to protect the fuel tank and fuel, cars usually have a fuel tank cover to lock, and only when they need to refuel will they open the fuel tank cover to refuel.