Air in cold water line from well. How do we replace the check valve?
You may want to call in a professional.  To get to the check valve, also called a foot valve, you need to pull the pump up by its connecting pipe, which takes a strong back and/or special equipment, and is VERY easy to mess up and drop the pump down into the well which is a big ($$$) problem!

But if you really want to try it, you'll need to rig up something so that you can hold the pipe as you reset your hands to make the next pull.  You will definitely need an assistant.  Just disconnect the electric wires, pull the pitless adapter release, and pull up on the pipe.  It will be HEAVY!  Be VERY careful not to let go once it releases from the pitless or down it will go.  Note that the well may be 100' deep but chances are the pump itself will be considerably less deep than that.

After you get some pipe above the top of the casing, most pros I've seen use a special tool that clamps the pipe to keep it from dropping down but some use a pipe wrench.  This is where it gets tricky as you need to unscrew sections of the pipe as you go to avoid trying to hoist 50' or more of pipe above your head.  Eventually it will bend and possibly break.

Once you get to where the pump is, you should be able to pull it out, remove the foot valve, and reverse the process to re-install the pump.

DISCLAIMER: This answer assumes that your well is installed using steel or rigid plastic pipe which is probably the most common in most areas until recently.  Note, however, that it may be installed differently as noted in the comment below.