Depends a lot on how they've been bent.

a bench vise, pliers and/or a hammer can be useful for getting them back in shape.

Whenever I get graphics cards with bent I/O shields, I take off the shield and depending on the damage, either straighten it out with pliers or bang it with a hammer on a flat surface to flatten it out again.

*EDIT*
Pretty much just said what weedeee said above 😁

momaka, Thanks for adding the part about taking the shield off of the board ! 😀

If there is nothing protruding in the way, a common practice is to put it between two pieces of wood before applying pressure with a vise or hitting it with a hammer. This also prevents heavy marks.