Editing on desktop
Click the "Edit" button to the right of the article to go to the edit page.
Editing on mobile
Tap the "Edit this article" button at the bottom of the article to go to the edit page.
To apply the changes to the article, you must first summarize the changes you made in the "Edit Summary" text box.
Check here to see what kind of notations can be used in the body of pixiv Encyclopedia articles.