Yes, you can upgrade your existing subscription directly in the shop. Don't forget to add the TV subscription to your internet order.
The remaining subscription period will automatically be credited to your new subscription when you activate Teleboy Home. The remaining amount will be credited to your first invoice.
Please note that Teleboy subscriptions purchased as inApp via iTunes cannot be automatically cancelled by upgrading to Teleboy Home. These must be cancelled directly with Apple.
You can find help on how to cancel inApp here: https://support.apple.com/HT202039