Two EAsy steps
1. On the old device, register your email with EA to get 100 free gems if you have not yet done so. This links your email to your PVZ2 account.
2. Install PVZ2 on the new device, do the email register again with the same email address, a code will be sent to your email address for you to confirmed you are the account holder, once this is done you will be given an option to load your old account onto the new device.
Please note when you are given an option to select accounts, do make sure you select the old one (usually it is the one with a lot more plants and in-game currencies) because if you pick a new one, your old progress will be overwritten and there will be no way back.