Deleting an installment with an invoice and payment already linked is as easy as creating them. You just have to complete the process in the opposite direction to creating these - just like is shown below. 

To unlink an invoice from a policy, go to policy's "Installment schedule" tab.
Now, click on "paid <paid date>" in "status w/customer" column.

If you click on the paid amount, you'll be taken this payment's page. From there, you are able to delete all linked invoices in "Linked invoices" section by clicking on the delete button. 

To cancel an invoice, go to "Invoicing" and click on "Invoices". From the list of invoices you need to locate the invoice that was created for this specific policy - you can also use the filter option and write the invoice number to find it more quickly. 

To cancel an invoice, simply open the invoice and press "Cancel invoice" in the upper right corner. 

Note that you need to locate and cancel the broker's invoice as well if it was previously created! To do that, locate the "Insurer statement" from "Invoicing" - > "Insurer reporting/billing". Now, go to your insurer statement's "Installments" tab and press "Remove installment from report".

You should now be able to delete the installment from the list of installments seen in your policy's "Installment schedule" tab.