Turn the light off and let it cool for a few minutes.
Get on the ladder and remove the cover to your fluorescent light fixture. Each fixture will have a different way to access it, but most will have a latch on one side that holds the cover in place. Flip the latch and the cover will swing down. Other covers wrap around the fixture, and you will have to pry the cover off of the sides.
Grab the T12 light that you want to remove and twist it until you can see a metal contact in one of the slots at the end of the light. Pull down on the fluorescent light to remove it from the slot. Once the first contact is out, you may need to wiggle it a little to get the second one out.
Remove the T12 fluorescent light from the fixture. On some fixtures, you may need to insert the contacts on one end of the light into small holes on the side of the fixture to be able to swing the other end down. You can then remove the light from the fixture.
Bring your T10 tube up to the fixture. Place one of the contacts directly underneath each slot on the ends of the fixture. If you had to angle the T12 tube to get it out of the fixture, you will have to do the same to get the T10 light in.
Push up on the T10 light to force the contacts into the slot, then rotate the tube to lock it into place.