Every feature in your data can be edited. You can edit attribute values and also feature geometry. To edit geometry open the feature detail and tap on the button (1)

Edit nodes position

The nodes of the feature are highlighted as green circles (2). The pink circle (3) is the only virtual symbol that can be used when you want to add a new node into existed nodes.  Tap and move this point to the desired location to add a new node. When you finish save the changes using the button (4)

Set exact location

You can move a node to a new location to edit geometry, but it's not ideal when you want to set exact coordinates. In such a situation tap on the node and choose the 'Edit location' option from the menu. You can set the new location by coordinates or from the current GPS position.

Undo, Re-do 

During editing, you can revert changes using Undo and redo buttons

Delete node

To delete node tap on feature node and from dialog select 'Delete'