OK, that's a tongue in cheek answer. The purpose of a stub page is to automatically redirect you to another page. That means once the page has been created you never actually view the stub page and therefore can't click on the edit button found below the content on every page. So a different approach is needed.
For a stub page associated with person page, using the "Search Wiki Page Names" box at the top of the right side column and enter the Given‑Name_Numeric‑Suffix of the page you want to edit and then hit the "Go" button. (For other stub page types, just drop the first few characters of the page name from your search term.)
On the resulting list of pages, you should see both the primary person page and also one (or more) stub pages. At the extreme right, against each entry will be an icon representing a spanner (aka a wrench).
Hover over the spanner icon on the search results page to reveal the "actions" list, which includes the option to edit the page.
NB If you type the exact page name, you will automatically be taken to that page when you hit the "Go" button, so you need to deliberately make the search ambiguous so that one or more results are returned. However, if your search term is too ambiguous, you'll wait a long while for the results and have to wade through multiple pages of results to find the one page you're looking for.