How do I edit a Wikipedia article?

I have been looking at the proofs in Book I of Euclid’s elements. I was impressed by the mileage that Euclid got out of the exterior angle theorem. His initial theorem, whose proof is fairly short, is that the exterior angle of a triangle exceeds either of the remote interior angles. From this Euclid proves the existence of parallel lines, that the larger of two sides of a triangle is opposite the larger angle, and the triangle inequality (sum of two sides greater than third).

I wanted to see if Wikipedia gave this theorem its due. I was sorely disappointed by the article. I do not feel qualified to overwrite the article, but I would like to post a box saying that more work is needed.