The Pickwatch Pro home screen is where you can manage your subscription and also find quick links to the great features. If you follow the link you'll be faced with this screen:

Update Payment Details

Clicking this button will redirect you to Stripe and here you can update your card details if you wish to change the card to be billed when your subscription renews.

Renewal Settings

If you click this you'll be given the option to turn auto renew off: