To edit a line's shape you first click on the line you want to edit. You will see white circles appear at the beginning and end of your line and at any line turns. You can then click on and drag any of these white circles to where you want. You'll also notice that there are semi-transparent circles midway between all the solid white circles. You can activate these semi-transparent circles by clicking on them once. This allows you to add more shape to your lines if needed. To delete a white circle click on it once.
See video below for a visual example.