Question: how to replace air conditioning duct?
Answer: prev. remove existing ductwork. attach second air duct to the first. Remove the Ductwork. Remove existing ductwork where needed. ... 
prev. put new vent in place. create wooden frame and secure in place. Create a Wooden Frame. ... 
prev. seal vent with mastic adhesive cement. fill in gaps with insulated duct board. Gaps and Sealing.