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

Edit nodes position

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

Set exact location

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

Undo, Re-do 

During edition you can revert changes using Undo and re-do buttons

Delete node

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