In most fish, the lower jaw is far more flexible than the upper jaw. Many fish have fixed upper jaws formed from the bony plates that also form parts of their faces.
The vast majority of fish have an upper jaw that does not articulate or that articulates very little when compared to the lower jaw. There are some exceptions, including some types of shark, that have hinged upper and lower jaws. Creatures such as the hagfish have no jaw at all, while catfish such as the plecostomus have jaws with almost no range of motion compared to other fish.