To edit a line feature simply select it on the map and tap 'edit' at the bottom center of the map view.
In edit mode, you can select which node you'd like to move. Simply select a node then pan and zoom the map to reposition it. You can use the arrow buttons to cycle through nodes on complex features.
In addition to the main nodes there are also midpoints. These are displayed inbetween two nodes and appear smaller than the main nodes. Tapping a midpoint will create a new node that you can then place independently. Use this method to add more detail to your feature.