Downgrading OS X (El Capitan back to OS X Lion)

Did not use Time Machine to backup my Macbook Pro (mid -2010) prior to reformatting. Couldn't download OS X Lion v10.7 from Apple App Store (not available) but purchased from Apple online (download). In the meantime downloaded El Capitan but mac is chugging. Not sure how or if I can downgrade to OS X Lion v10.7.