Here, people in this forum arent nice enough to help you so i will.
function doa180()
aim1 = LocalPlayer():GetAimVector():Angle()
LocalPlayer():SetEyeAngles(aim1 - Angle(0, 180, 0))
end
concommand.Add("lenny_180", doa180)
Put it into a lua file and load it and write "lenny_180"