Don't try to index individual rows of the Edges table to modify properties of the edges. [You could view individual rows of the Edges table using indexing as shown in the "Add Edge Weights" example on this documentation page but you can't change them.] Access the variable inside the table then index individual rows of that variable.
G.Edges.Value(edgepath, :) = [2, 2]
G.Edges % Show the updated table
If you were doing this just so you could increase the width of the lines used to draw those edges, you could use the highlight function on the plotted graph instead.