How do I change the order of my categories in the main menu?

Go to Products > Categories.

Hover over the category you wish to move. The background color of the category will change to gray and your cursor will change to a four sided arrow.

Click on the category and while holding your mouse button down, drag the category up or down in the list. You can also drag the category right to make it a subcategory of the category above it or drag it left to make it a parent category.

Note that category changes are saved automatically but make take a few minutes to display on your website due to caching.