Hello!

I have a dimmer at the top of the stairs (pictured) that has two black wires and a red. The middle wire (black) is capped and not connected to anything. There is also a ground in the box, not connected. Can I replace this dimmer with a switch? If so does it need to be a 3-way?

At the bottom of the stairs is a 3-way switch, with a white, red, black and ground. the white wire is on the black screw.

Thanks is advance for the help!

If that center wire is capped, then it does look like a three-way switch that is wired up as a two-way. Like in your fan / switch post, just check to see if there is another switch that operates the same fixture as this one. If so, then it is a two switch system - but it may be wired incorrectly in an attempt to put a dimmer into the system.