How can I change the angle of an edge loop? How can I change the angle of an object like at 19:44 of https://youtu.be/mu9J0D6ebVw?t=1167 when he adds wingtips?
I think you mean Shear? Shift+Ctrl+Alt+S with selected vertices, then drag the mouse cursor around to effect the amount. 



But I think he uses a simple Rotate (R), with the pivot point set to Median Point, then move the mouse to adjust the rotation amount.