Changeover Point. Explain please?
 

So I did read the definition:
"Point between two stations is indicated by mileage from the station to the point of change".

I see the little picture of the COP on the airway chart with a number saying 21 and another number saying 32.
I don't understand exactly what it is, somebody please explain?

At 21 DME (or NM, depending on what's written on the chart) from the station behind you, stop flying along the outbound radial from that, changeover your VOR/DME/TACAN to the station ahead of you, and start flying the inbound radial to that. You'll be 32 DME/NM from the one ahead when you change over.

They indicate the point (DME distance) at which you should change over from one VOR frequency to the other.
If it is not indicated, than you should change frequency midway between the VOR's.

The previous posters are absolutely correct. Just to add some detail though...
The COP is not half way between the two because one transmitter may be more powerful than the other, or at a higher elevation or have more / less interference. The COP is where the strength of the two is equal at a defined altitude.

And how would you then define change-over point from Jeppesen En-Route Chart for a VOR to VOR airway if there is no change-over point/distance specified?

If a COP is not defined then you do use the mid way point.

Sure, but by default the COP is halfway between stations - only if the transmitter power/obstacles/.... are significant, then it won't be halway between stations - not the other way around.