The elektrotanya website link proposed in the other solution here did not work for me. So with a similar sticky keys problem on my YDP-113 yamaha digital piano, i just started taking it apart.
I found that the problem was that the sticking keys actually had small cracks at the hinge end. See attached photo for a picture of the hinge end of the key. The cracks are very small and hard to see, but if this is your problem, you will see it in a good light. Having the key apart will also allow you to clean the sides well in case there has been a bubble gum attack or similar problem.
After learning that replacement keys are extremely expensive (like $15 per key...) I decided to move the bad keys to the highest and lowest notes on the keyboard (88 key keyboard so there is some seldom used territory). The center several octaves are now free of sticky keys and this is good enough for now.
The procedure on my YDP-113 was:
1. Remove black screws from back panel of unit to allow top to slide forward and then lift off. Set aside.
2. Roll the roll top all the way closed and then simply lift up to remove. Set aside.
3. Remove the six screws which attach the roll top gear rail (3 screws on each side panel). Set aside the gear rails.
4. Remove 2 screws in the key top bracket, and 1 screw from each end of the key top. "Key top" is my name for the small strip which lies along the top of the keys. If you have an official or better name, please share.
5. Remove the 10 or so large screws and two small screws which hold the keyboard panel down in the unit.The keyboard should now be able to slide a few millimeters toward the back of the unit.
6. Remove 1 screw to take out the power switch panel. The front of it hooks in with a tab, so slide it back and then it will lift out.
7. Careful now. I felt this was the one operation where I had an opportunity to ruin the unit if I would have fumbled: Lift the keyboard up slightly and rest its front 'legs' on the front edge of the case. Careful of small wires to not
rip them. Careful of the circuit boards that are underneath the keyboard frame.
The objective here is to allow the keys to come out toward you so that they can be removed and replaced or swapped. You can kind of see in the cracked key end photo, how the keyboard is up and partially out of the unit.
8. You now can do the actual swapping. To remove a key, pry up very gently in back of the key with a small screwdriver.
It will pop up. Then gently work the key around to free it from the return spring without bending that spring. Replace with a new key or swap will a good working key in a little used position. Note that B and E are interchangeable, also C and F. Some other keys also look like they could interchange though I didn't personally try it.
9. Reassemble in reverse order of disassembly.