Wire the switch with Common to Gnd, NO to one input, NC to the other.
Use the internal pullup resistor on both inputs.
When the switch is to one side, one pin is grounded (LOW) and the other is pulled up to 5V (HIGH), and vise versa.
You only need a SPDT switch, not a DPDT switch. If you have a DPDT, you can use just one side of it.