To change the device you want to play on, you'll need to go to your profile (the farm house) and set a username and password. You can later change it later with the "Change Password" option.
To switch accounts, open My Free Farm 2 on your new device and select "Switch Account" in your profile. A window will pop up which will allow you to enter your username and password. Confirm with the "Select Account" button and you're all set!