You must have "Full Access" or "Limited Access - Financials" permissions to edit invoices.
How to Void an Invoice
An invoice can only be voided if no previous payment has been made. If the invoice has a full or partial payment, the open payment(s) must be "credited" to $0. It will show as canceled, not paid.
From the left navigation menu, click Financials > Invoicing.
Find the desired invoice, and click the Sale ID.
In the top right corner, click 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.
Click Apply.
How to Refund an Invoice
You can issue a refund for an invoice from Invoices or the Payout Report.
From the left navigation menu, click Financials > Invoicing.
Find the invoice you'd like refunded and click the Sale ID.
In the top-right, click Issue Refund.
Choose the payment method to refund, then enter the amount to refund.
Select Yes or No for Do you still need to collect this money in the future?
If No, add any notes and click Refund.
If Yes, add any notes and click Next.
Enter the date to recollect the refunded amount.
Click Refund.