All expansion cards are secured to the case in some way to prevent the card from coming loose. Most of the time this is accomplished with a retaining screw.
Remove the retaining screw and set aside. You will need this screw again when you reinsert the expansion card.
Note: Some cases do not use retaining screws but instead feature other ways of securing the expansion card to the case. In these situations, please reference your computer or case manual to determine how to release the card from the case.