The pay invoice early option can be a bit difficult to find as it only shows up within your Organization's Settings area within the "Delete organization" section's "View checklist" area once you have completed a few prerequisite steps. Unfortunately, the ability to pay an invoice early doesn't directly show up within the Billing area.
In order to be able to pay the current month's pending / month-to-date invoice you'll first want to ensure that if you have any databases on a Scaler Pro plan that those have been deleted first as that will end their plans and prorate the fees on the current invoice.
Databases can be deleted within the Settings page for each database.
Once that's been completed, you should be able to go into your Organization's Settings page and can scroll down to the "Delete organization" section and expand the "View checklist" area as shown below.
If the items listed have been completed already you should be able to see the "Pay invoice" button shown in the example below which will allow you to pay your invoice early, rather than waiting until the end of the month when invoices are normally automatically processed.
If this is your last / only Organization the system you may encounter the "You must belong to at least one organization." message and may want to proceed with deleting your user account instead which can be completed from the Account Settings area: