How do I change the navigation look for my flex website?

With the Flex Manager section of your account, click on the Flex Website you would like to manage from the list of websites under your account, then select NAVIGATION from the side bar

FlexSite Menu

From there you can select the type of main navigation bar you would like to use for your Flex Website

FlexSite Navigation

Each theme of Navigation is collapsable and tablet, mobile, or desktop friendly