This might be tough to explain, but depending on the type of faucet, you may simply be able remove the handle, turn the knob 180 degrees and reinstall the handle.
Now that sounded confusing.
Anyway, when I built our house, I installed all delta posi-temp valves. The valve has a single knob that turns just under half a turn. It starts cold, then the farther you turn it, the hotter it gets. When I replaced the shower valve in the motorhome, I installed the same valve since we like it, and know how to use it. Got it all plumbed up, and wouldn't you know it....reversed hot and cold. Well, it turns out, when I installed it, I had turned the shaft, so then I installed the handle 180 out of phase. On those type of valves, the shaft can rotate continuously, but the actual stop for rotation is in the knob.
Try removing the knob from your shower, and grabbing the shaft with your fingers or a pair of pliers. Turn the shaft, and you'll see the water turns on, then once the shaft is turned a half revolution, the water will turn off again. If you install the knob at that point I think you find the cold water coming on first.
If this doesn't work, you've got a different type of knob, and just ignore my ramblings...