Removal of managing director of a pvt. ltd company

Dear Experts,

There is a Pvt. Ltd Company, whith three share holders A (18%), B(5.5%), & C (76.5%). All are full time directors in the said firm, while "A" is the Managing Director since last 20 years. Now "A" & "B"have crossed 70 years od age.

My question is how to remove "A" from the position of MD to a non-executive director, and simultaneously appoint "C" as the new MD? "A" may not like this and may offer resistance, while "B" is cooperative. Please provide me with the correct legal procedure to do the same.

Thanks,