Can I edit a page's lines?

Yes. Click "Edit": Draw, Line (close a gap), Erase, Fill space, Remove mark, Color, the brush Size (0.75 to 16 pt), Undo and Redo (Cmd/Ctrl+Z, Shift+Cmd/Ctrl+Z), Thicken lines, Show small spaces (in red), Zoom (Fit, 100%, 200%), Save the page or Cancel. Saving checks the page again, then colors and numbers it. An edited page keeps your changes and its colors when you draw the rest of the book again.