I placed the arrow and as is typical in drawing environments like this, I got a handle to use to rotate the arrow (this is the green G like thing you see on the canvas - highlighted by the red ellipse). The neat thing is you can drag the handle so that the further away you drag the handle (indicated by the thin blue line in the UI), the finer is the rotation so you can decide how much control you want over the rotation. Compare this to Powerpoint (which is a pretty good regular use graphics program for me) and the rotation is not as intuitive or easy and requires very careful mouse manipulation to rotate things right.
This does have a problem that you are limited by the canvas and in a rectangular canvas if your rotation arc (after you have extended it to almost the edge) passes over the edge, the rotation stops. But it seems to work well - give it at try.