Turn off the cold water supply.
Remove the cold water inlet.
Assuming you have a new dip tube on hand you will notice it is connected
to a galvanized nipple. When you remove the existing one, your
wrench will turn it out counter-clockwise. Place another wrench
on the hot nipple so it opposes the force of the wrench on the
cold nipple. Teflon tape the new nipple. Your away.