How do I replace individual textures in hammer?

Sorry to sound like a complete noob, but I’m making a CS:GO map and I don’t know how to replace individual texutres? Anyone have a solution?

I’m stuck with this at the moment:

It’s for left 4 dead, but the process is the same for any source engine game.

Thanks a lot, this solved the problem exactly!

Next time use the mapping question megathread.

[editline]what[/editline]

What? Why not?