You can downgrade through iTunes. Download the IPSW and put phone in DFU mode and then Alt + RightClick on Update iOS in iTunes when it you plug the device in.
Only to the iOS versions currently being signed, which doesn't provide a huge selection. https://ipsw.me/ provides a nifty way to see what is available.
FYI, ipsw.me has been having some troubles lately, and the reported signatures might not be accurate.