With the exception of a few protected pages, every page has an "Edit" tab which lets you edit the page you are looking at. It is the Scratch Wiki's most basic feature, and allows you to make corrections and improve articles.
To practice editing, go to the sandbox and click the "Edit" tab. That will open an editing window containing the text for that page. Type in something fun and interesting, or just replace words for your own words. Now click the Save page button and see what you have done!
Your first practice edit (above) left off two steps that you should do if you are editing an article or another page that will be read by other Scratchers. So, click the "Edit" tab again, enter some text, then do those two additional steps.
First, any time you edit a page, it is considered good etiquette to enter an explanation of your changes in the Edit summary box, which you'll find below the edit window. It's okay for your explanation to be quite short. For example, if you are making a spelling correction, you might just type "typo". Also, if the change you have made to a page is minor, such as correcting a spelling or grammar error, it's helpful if you check the box "This is a minor edit". For your "sandbox" edit, you probably want an edit summary such as "Testing".
Second, you should always use the Show preview button. After you've entered a change in the edit box for the sandbox, click the Show preview button instead of Save page. This lets you see what the page will look like after your edit, before you actually save. We all make mistakes; this feature helps you catch them before other people see them. Using Show preview before saving also lets you try format changes and other edits without cluttering up the page history. Do not forget to save your edits after previewing, though!
Save the page
Done the edit summary? Previewed the page? Then you're ready for the final step: click the Save page button.