Editing geometries

Make the layer with geometries that you want to reshape selectable.
Inspect the geometry whose shape you want to change, using the Mark & Move tool.
Click Edit record button on the right pane.
Click on Edit nodes to show the points in the geometry which will allow you to move them or delete them.
Or, click on Reshape to add extra shapes to the original polygon and then use the merge tool in order to append the shape.

With the help of the merge tool two separate polygons can be combined into one.