To transfer your subscription to a new device, first unhook your old device on the ForeFlight website.
Once signed in, click "Devices" and then click "Remove" next to the old device.
After you have removed your unwanted device, here are instructions on how to install and activate ForeFlight Mobile over Wi-Fi:
If needed, you can reset your password within ForeFlight Mobile by going to the 'More' page and then tap on the 'Accounts' line on the upper left. Tap on 'Sign In' on the right side of the page and then enter your account email address. Once you have done that, tap on the 'Forgot Password?' button. A new temporary password will be emailed to the email address you entered.