These steps assume you are set up as a Billing Admin with permission to add client accounts to your accounting firm's monthly bill.
Step 1
Navigate to Settings->Billing
Step 2
Select the 'Upgrade or Downgrade' button
Step 3
Select 'Change Plan' and pick the plan you wish to move to
Step 4
Select 'Update Subscription'. Your plan has been changed and will be reflected in the following month's invoice.
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