From your dashboard, click on "Billing" on the left-hand side then select "Invoices" from the drop-down.
From here, you will have two ways of editing the amount of the invoice. Note you can use the filters at the top of the page to find the invoice you are searching for.
From this list view, click on the amount under the "Amount" column and manually edit the number here. Note: you must click "Save" for each individual amount you change.
Option 2 (Better for adjusting those on monthly payments):
Click the account ID # under the "Account ID" column next to the invoice you want to adjust. From this view, you will see all invoices associated with this player. Here you can adjust the amount for any invoice you want to alter by clicking on the amount. Note: you must click "Save" for each individual invoice you adjust.