How do I replace this dimmer switch to a conventional switch?
That is a 3-way dimmer, so you'll have to replace it with a 3-way switch.  The new 3-way switch will have 4 terminals (screws), 1 black (Common), 2 brass (Travelers), 1 green (Ground).

When working with 3-way switches, the Common terminal will always be the odd color (odd man out). For example, on your current switch the Red wire is the Common. On a regular 3-way, the black (odd color) screw is the Common.



To prevent confusion, it's often a good idea to work with one wire at a time. Rather than disconnecting all the wires from the old switch, then trying to remember what went where.


Start By turning off the power at the breaker, and verifying it's off with a non-contact voltage tester.
Disconnect the bare copper wire from the old switch, and connect it to the green terminal on the new switch. 
Disconnect the black wire (the one connected to the red wire on the old switch), then connect it to the black (Common) terminal on the new switch.
Disconnect the black and red travelers, and connect one to each brass terminal on the new switch.
Turn the breaker on, and verify proper operation.
Turn the breaker back off, and mount the switch in the box.
Turn the breaker back on, and again verify proper operation.