Remove the nut, then remove the bolt, cut the wire end off, take the bolt off the wire.

Does the brake open on its own and close by hand? If not, lubricate it and work it until it does. If it won't, replace it.

Grab the end of the wire with a vise grip, pull: does it pull the lever tight? Will closing the brake lever pull the wire into the cable? If not, lubricate the cable with graphite; if that won't loosen it you may need a new wire and/or cable.

Walmart sells something they call a 'repair kit' that has a set of derailleur and brake cables and wires for about $6. That's the cheapest deal in town, just for the wires. I have a bunch of spare cables in my box of spare parts.

These are steel. They can rust. I've fixed up friends' old unused bicycles and found the wires and the insides of the cables rusty. If you really want to ride a bicycle replacing them is a negligible cost.

I dress the ends of my cables with 'Goop' ('cause that's what I have around) and, occasionally, a bit of shrink tubing (unshrunk) (same).