You can measure the angle of an object by activating the tool  (Angle measurement). You now have to click three times to set the angle and position of the measurement. 

You can not edit the measurement once it has been made, but you can delete it and try again.

To delete a measurement you have to activate the measurement by activating the (Select) tool. When it's activated you just have to click on the blue line of the measurement and press Delete/Del on your keyboard.