If your account is linked to an upjers account, you can simply log back into your account in the game settings - the gear in the bottom right corner:
Just pick the option Log in with upjers account:
(Remember that the name of your upjers account is not necessarily the same name you had in the game - if you can't remember your upjers account name, you can check the registration e-mail you got when you signed up for the upjers account)
If your account was never linked to an upjers account, we may still be able to restore your progress for you. To do that, we'd need the support ID of your old account, and the support ID of the new account you wind up in (for this to "stick", you'll need to play at least up until the panda cutscene).
You can find your support ID in the game settings :
If you no longer have access to your old device/account, we may be able to find it with a username (if it was something unique) or with a transaction number from the Google Play Store if you've ever made a payment for the game. You would find this in the confirmation e-mail for the purchase, or in the Google Payments Center (payments.google.com). It would look something like this: GPA.1234-1234-1234-12345.
Just send us a ticket with the old and new support ID, and we'll swap them out for you!