Since most turns are done at low speeds, your vehicle should be driven in the gear that corresponds with that speed—usually 2nd when not required to stop and 1st when you are.
Taking a normal turn in too high a gear, say 3rd or 4th gear over-burdens the engine at too low RPM and over time can cause serious damage to both the engine and transmission.
Prepare for turns well in advance by slowing early enough to complete the downshift prior to turning – not during the turn itself.
As you approach the turn if you’re already in 2nd gear then you’re all set to turn. You don’t have to shift down just for the sake of shifting. Shift only if you’re not already in 2nd gear.