An instrument for locking and unlocking a head of a bone anchor relative to a receiving part of a polyaxial bone anchoring device includes a tube assembly with an inner tube and an outer tube at least partially around the inner tube, and an actuator assembly including an actuating mechanism movable from a first orientation to a second orientation to displace the inner tube and the outer tube relative to one another between a first axial position configured to unlock the bone anchoring device such that the head is pivotable relative to the receiving part, and a second axial position configured to lock the head. The actuator assembly is adjustable between a first configuration where the tube assembly assumes the first axial position when the actuating mechanism is moved, and a second configuration where the tube assembly assumes the second axial position when the actuating mechanism is moved.