I have small white particals of "grit" in my sink faucet washer and need to clean it about every two weeks. Monday morning, my shower faucet head started spraying towards the ceiling from the sides. Turns out a seal had been blown out and guess what... small white particles had clogged the shower head (there was no screened washer in the unit). Someone suggested that I need to flush the water heater. I tried to locate a manual for this unit RN0894121397 Model21V40-7N but failed. Any suggestions before I search YouTube for generic how-to videos?

You have a faulty dip tube which is deteriorating into white sand like particles. Replace the dip tub and power flush the heater for at least 5 minutes. How to Repair or Replace Defective Water Heater Dip Tubes | The Family Handyman

Solid advice Bob!
Spopo, If those little white particles can't be crushed by rubbing between your fingers they are m indeed, chunks of the dip tube, However, If they can be crushed they are calcium carbonate and not a dip tube. A dip tubes function is to direct the cold water down to the bottom of the tank where it will be heated by the hard working bottom element. If the dip stick's broken you will go from hot to luke warm in short order.
Have you noticed a shortage of hot water lately? As you can see by the image I've put up the dip stick is located under the cold water inlet. If you replace it yourself Do not put heat or solder directly over the cold water inlet The tube will melt and drop if you do. Instead either use a 5X3/4" brass nipple with a female adapter or make up a mail adapter and solder a 6" piece of copper and let it cool down before installing.
By now you should have flushed the heater before you have to open up and clean out every faucet in the house Let me show you how.
For long life and fewer troubles you should keep your heater clear of mineral build-up by FLUSHING NOT DRAINING on a regular schedule.

Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes.

Do this monthly to keep it clear. Now flush out your hot water lines on ALL fixtures that are affected . Now pull each aerator and clean the screens. Be sure you put them back together the same way you took them out. Don't forget to flush it out every month. Your heater will thank you for it.
Hope this helps, Tom