It depends on the arrow type. For Dubie's line type arrow you can use the rotation controls as shown or click on one end and drag it to a new position, creating a new angle. In Camtasia 2018, you can add text to arrows and rotate them using the "bubble" controls on the edges. The extra bubble next to the central one is a turn control (black arrow in screenshot). Click and drag it in a circular motion to change the angle. The the text realigns with the angle of the arrow.