You are unable to amend a payment plan when the order has been placed. Once you have purchased a payment plan you can not then pay it off in a one-off payment. You have to pay monthly for the length of time agreed upon on purchase.
It’s also not possible for you to specify a specific date for payments to be taken or for the payments to be taken early.
You must ensure that you have the required funds on the first day of each consecutive month the payment plan runs for.