In order to change existing plan or edit payment method you have to be logged in as the organisation owner. Once you have that access go to Settings > Plan and billing details. There you will find all details regarding numbers of users you are paying subscription for, number of partners that have active Partner Portal access, number of Phones rented and next billing invoice date and credits for additional services.
To change existing details click on "Actions" button in top right corner and you will get dropdown menu with options to:
Change main subscription plan
Change main subscription method
Cancel main subscription
After clicking on "Change main subscription plan" you can scroll down the page and select plan you would like to subscribe to:
After clicking on "Change payment method" you will be prompted to add another card details:
After clicking on "Cancel your main subscription" you will unsubscribe from Salestrekker and your org will be active until next invoice date and after that will show "Organization suspended" message until you activate it again.