Can I overvolt this flashlight-Lowes
 

I recently bought a Lowes flashlight, Task Force, with a Cree emitter. It uses 2 C batteries, which put out almost 3V at max (2.4V with the included batteries-alkaline.)

Can I drive this emitter with 3.6 to 4.5V, without doing any damage to it???.

Most likely your flashlight has a constant current driver circuit. Overvolting the driver would, at best, have no effect on the brightness of the light. At worst, it will damage the driver resulting in no light. There's no way to tell without knowing the specifications on the driver circuit.