That is a steel lintel that takes the load of the bricks above and spreads that load to each side of the door. You will have that over the windows too.
You won't be changing that piece, it is part of the structure, it wants to be painted with a paint for steel.
I've done many of those over the years. If it's rusty, clean it up, use rusty metal primer, then top coat with an oil base paint in a color of your choice.
Easily? Sure, provided you chisel out those 6-7 bricks up top, find exact duplicates, and wedge them back into place. Not my idea of easy just because of a bit of rust.
You do not want to replace it. Sand or wire brush it and paint with rust paint like tremclad or rustoleum.
This is probably what you have up there. Its quarter inch thick hunk of steel angle weighing about 25 pounds. A little surface rust ain't going to make it fail. As others have mentioned, just go over it with rust paint.