Individual subscriptions allow you to sign in to ForeFlight on 2 iPads and 1 iPhone at once, so if you've already reached the maximum number for each device then you'll need to remove an older device from your account. On the other hand, if you're signing in to ForeFlight on a second iPad or an iPhone for the first time, you can just download the app and sign in without removing any devices.
NOTE: Do not use an iCloud backup to add ForeFlight to a new device. Restoring the app from an iCloud backup can result in problems that render the app inoperable, requiring you to delete and re-install the app from the App Store.
Removing Old Devices:
To remove an older device from your account, sign in to ForeFlight's account management page at foreflight.com/account. In the Devices table at the bottom of the page, click "Remove" next to the device that you want to remove from your account.
You can also remove devices from your account inside ForeFlight Mobile by going to More > Account and tapping "Remove" next to the device you want to remove in the "Also Signed In On" table at the bottom of the right column.
Adding New Devices:
After you have removed your unwanted device, follow the instructions here to install and sign in to ForeFlight Mobile over Wi-Fi.
If needed, you can reset your password within ForeFlight Mobile by going to More > Accounts. 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.