To edit an existing pattern in Illustrator CS6 or later, just double click on the pattern in the Swatches panel.
You're then placed into pattern editing mode in Illustrator, where you can make changes and save iterations on the pattern design. This mode is similar to Isolation mode, where you can work only on the elements that make up the repeating pattern. The Pattern Option dialog also appears, that gives you controls over the pattern tile size, tile types, etc.
If you need to learn how to create your own patterns from artwork, just do a web search for tutorials as there are many on working with the new pattern tool in AI.