It appears to me that the title of this page must be changed from "Euclidean Division" to "Euclidean Division Lemma". So, how can I do this given that the "edit" option of Wikipedia only allows to edit the content of the page and not its title.
To rename a page, you need to "Move" it.
The Move link can be found under the "More" menu (assuming you've not customized the user interface). Alt+Shift+M is the keyboard shortcut.
The advantage of moving the article is that it leaves a redirect behind, so that all of the previous links don't break.
See: Wikipedia: Moving a page for more information.
While Wikipedia encourages people to "be bold", you'd do well to discuss the change in the article's talk page first, otherwise you may find your move quickly reverted. Especially since I see several previous discussions about the name of the article that settled on the current name as a compromise.