How to Void an Invoice:
From the left navigation menu, click Financials > Invoicing.
Find the desired invoice, and click the Sale ID.
In the top right corner, click the three dots > Void.
Verify by clicking Void.
How to apply a credit to cancel future payments:
In the top right corner of the invoice, click the three dots > Apply Credit.
Add enough Credit to each installment to zero them out.