Why don’t I have a slider to change the winds aloft altitude?

First, check that you are running ForeFlight Mobile version 6.7 or later.  Here are instructions for updating your app to the latest version:

    www.foreflight.com/support/app-update

Once you have the latest version installed, go to the Maps page and select the Maps Layer drop-down selector in the upper left. Select “Winds Aloft”.