Best way to seal a chimney

[larry]

Advice welcomed!

I have four fireplaces in our house, two upstairs and two downstairs. Downstairs are sealed off with some ply with a ventilation grille built in. Two are old fashioned painted fireplaces as in attached picture. 

Working on improving thermal efficiency of everything and thinking about the best way to seal the chimney short of knocking it down. 

I know there are various products on the market, but what works best? 

Also how on earth do I open up the upstairs fireplaces?