Any Administrator can go to Setup-->Billing and make changes to your account there:
You can change your current plan by adding products or adding user licenses as well as update your payment information. You will also be able to view any past invoices via the Billing page.
You can also set up candidate texting within the Billing page:
Selecting "Setup" will bring you to the following screen, where you will add your area code, select "Search numbers" and select an available number for use within your ApplicantStack account:
Once you read and then select "I Agree", you will notice your new plan listed on the billing page:
For more information on texting, see: Texting
If you have further questions please contact your account manager or create a ticket via the Help button or click here for all questions.