Help my navman isnot charging . ineed to send it off 4 repair. where and how

Check the charger with a multimeter for voltage on the charger lead. If there is no voltage then this is as fault.
You should return it to the dealer if still under warranty.
If the fault is within the F20 then you should definitely take it back to where you purchased it. If it is still under warranty then the dealer should replace it free.
If this is not possible and because you did not specify your country you will have to go to a search engine ie www.google.com and type in "navman repairs" Paris. Include the quote marks and substitute Paris for your city.
When you are sure of a good result with an AUTHORIZED Navman service center, go to the repair center web page, find their telephone number and ask them if they are able to repair your device.
If you are not able to take the device to the center then tell them you will be sending it by your postal service.
Just wrap it up well in a box, preferably the box it came in. Include all the parts. And send by recorded post from your post office. Insurance is recommended and usually quite cheap.
Good luck, and if you want me to do the searching for you then come back and comment with your Country and city, I'll try to find a repair center near you.