Picture of How to sharpen a Norelco electric shaver
Replacement blades are expensive,$30 to $40.  Why not save some money and sharpen the old one ? 
Back before safety razors and during world war II, when steel was scarce, people would resharpen their double edge razor blades on a sheet of glass.  I figured why not try it on my Norelco shaver that was leaving my face raw,  and it worked,   Here is how I did it...

You will need a small brush to clean shaver and heads, also a flat piece of glass ( a mirror or  picture frame glass will do).

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Step 1: Remove shaver head

Picture of Remove shaver head
Brush out any hair in the body, do not use water. 

Step 2: Remove cutters from head

Picture of Remove cutters from head
Rotate the white wheel to release cutters and remove from housing.

Step 3: Clean housing and cutters

Picture of Clean housing and cutters
Use the brush and running tap water to clean the housing and the cutters.  Close down the drain in the sink, you don't want to drop anything down the drain, then slide out only one of the cutters.

Step 4: Seperate cutter from shield screen

Picture of Seperate cutter from shield screen
Again clean the cutter and shield screen with brush and running water, junk gets trapped in the cutters.

Step 5: Sharpen blades on glass

Picture of Sharpen blades on glass
Place the blade cutter on the glass.  Here is the gross part...spit on the glass, put your finger on the top of the cutter with light pressure slide it into the spit and start moving it in a figure eight.
 Continue the figure eight motion 50 to 100 times and towards the end be more gentle with the pressure.  Always keep the glass wet.  I tried water but the spit works best, it has a lubricating quality.

Step 6: Replace cutter

Dry with tissue and replace cutter in shield screen, then remove the next cutter and sharpen it.
Do not remove all the cutters at once as I don't think it is a good idea to mix them up. 
Reassemble everything and enjoy a great and painfree shave.