Without the model number it's hard to give exact advice, but generally most chimney sections are fixed by a hidden bracket right at the top where the wall meets the ceiling. There should be a small screw on each side right in that top corner. Take both screws out and the chimney should telescope down enough to pull towards you and detach from hood. After installation the screws may have been covered by cove or mastic which could make them hard to see.
Chippy, June 2016