Directly spraying contact cleaner into the top of the key switch, without taking the switch apart, is the fastest method but the cleaner may not reach the contacts it's supposed to clean. Before spraying, clean the area around it a little. With the keyboard live (or with the key contacts connected to an ohm-meter) use the plastic tube which came with the spray to squirt cleaner so it will get inside the key switch. Try to move the key push rod up and down while spraying. Don't let the cleaning liquid get under nearby keys where it may pick up dust and then seep (with the dust) into adjacent key switches. If you make this mistake you may fix one key but damage nearby keys. If this should happen, immediately work (repeatedly press) the affected nearby keys until they continue to work OK.
You might tilt the keyboard so that the cleaner flows better into the contacts. For the CIT101e terminal with an Alps keyboard, this means tilting the top row of numeric keys up toward the ceiling. While moving the key switch up and down with a pen or small screwdriver handle avoid getting the toxic cleaner liquid on your skin (or wear gloves). You might try turning the keyboard upside-down while working the key to drain off remaining cleaner. The more cleaner you squirt in the more likely it will fix it but it is also more likely to do more damage to the plastic or contaminate adjacent keys, so use what you think is just enough to do the job. Once the key works OK, work it up and down a little more and test it a half minute later, etc. to make sure it will still work OK.
Sometimes a key works fine when the contacts inside are saturated with contact cleaner liquid. But when the liquid dries a few minutes later then the resulting scale deposit left from the evaporation of the cleaning liquid on the contacts, prevents good contact. Then the key may work erratically (if at all). Operating the key when the liquid is drying inside may help. Some switches have the contacts nearly sealed inside so little if any contact cleaner reaches the contacts. The cleaner that does get to the contacts may carry contamination with it (cleaning around the tops before spraying helps minimize this).
If you want to disassemble the key switch, first inspect it to see if it comes apart (and if so, how). Sometimes one may remove the cover of the switch without removing the switch from the keyboard. To do this pry up (or pull up) the top of the key switch after prying apart thin plastic tabs that retain it. You may be able to use two small screwdrivers for this and be able to pry up the switch while prying apart the plastic retaining tabs. Don't pry too hard or you may break the plastic. If this can't be done, you may have to unsolder the switch and remove it in order to take it apart (or replace it). Once the switch has been taken apart you still may not be able to see the contacts if the contact surfaces are sandwiched together (nearly touching). You may put contact cleaner on the contacts by squirting some cleaner on an edge so it can penetrate onto the contacts. Insert a tiny screwdriver blade just a little so as to pry apart the edges as you apply the cleaner. This will help the cleaner reach the contacts. Work the contacts open and closed with a screwdriver to help clean them and note if the key is working by looking at the terminal screen.
There may be some kind of clip holding the contact surfaces together which needs to be removed so you can pry them apart. Take care not to loose small parts as they may fly up into the air when taking apart a key switch. As a last resort, you may try bending the moving part that the push-rod pushes so as to make a stronger contact. In my terminal, this part looks like the electrical contact but it just pushes the real electrical contact thru a thin insulator.
When putting the key switch back together, make sure that the spring is in the right place. If, after you assemble the switch, the key pushes down too hard or too easy, the spring is likely not positioned right. If the spring is supposed to be recessed into a hole on the push rod, one way to temporarily "glue" the spring into the push-rod is to use a half-drop of water on the end of the spring. Then insert this end into the push rod and assemble quickly before the water dries. This should keep the spring from falling out of the push rod during assembly.
