Did not see anything useful on the internet. if you can't find it … buy a gallon of 50/50 and dump some from the lower hose.
Fresh coolant is more important than a flush on a 2016.
When I drained our 2015 Sonic , I dropped the lower radiator hose , re- connected it and filled it with 100% Dexcool , from Walmart . I never buy 50-50 , unless I mess up and grab the wrong jug .
You didn't specify which engine, However looking at the Rock Auto pic linked with what's supposed to be the OEM radiator for the 3.6L, there appears to drain plug/valve/screw on the left side below lower hose. Alternatively as noted, removing the lower hose an option.