Changing auto-generated letters on polygon points

[Susanna]

I am trying to copy a diagram in an assignment sheet so that I can highlight various elements of it in my answer.

The letter labels of the points of the shape that I am trying to draw seem to be generated automatically when I draw out the project but these don't correspond to the letters in the diagram. If I try and add text and then delete the auto-generated letter then that deletes the whole line.

How can I change the auto-generated letters at the nodes?