Rigidbody.AddTorque
AddTorque(torque: Vector3, mode: ForceMode = ForceMode.Force): void;
Description

Adds a torque to the rigidbody.

As a result the rigidbody will start spinning around the torque axis.
	// Spins the rigidbody around the global y-axis

function FixedUpdate () { rigidbody.AddTorque (Vector3.up * 10); }
If you want to apply a force over several frames you should apply it inside FixedUpdate instead of Update.
AddTorque(x: float, y: float, z: float, mode: ForceMode = ForceMode.Force): void;
Description

Adds a torque to the rigidbody.

As a result the rigidbody will start spinning around the torque axis.
	// Spins the rigidbody around the global y-axis
	function FixedUpdate () {
		rigidbody.AddTorque (0, 10, 0);
	}