If you are paying an installment plan for past taxes, that has nothing to do with the amount of tax you owe or refund you get on a current tax return. However, if you are due a refund for a current return, it will automatically be taken and used to reduce the debt you have been paying in installments. It does not reduce the amount of each installment due, but will reduce the total number of payments and the length of time you have to pay.
Or, if I misunderstood your question, please give additional details.
