How do I change the order of posts?

By default, WordPress sorts posts by the publish date. The most recently published posts appear first, while the older ones appear later. However, you can easily change the publish date of any post, by editing the "Publish" panel on the right, as this screenshot shows:

Note also: The same logic applies for custom post types. Depending of the theme you use, such custom post types might be Portfolio items, events, or even slides. etc.In any occasion, changing the publish date will change it's order at the front end.