Generally, once you have reported the installment sale on your return, the Gross Profit Percentage doesn't change over the life of the installment payments.
However, if you are entering this sale in the current tax year, you can return to the installment sale section and revise your entries.
Gross Profit Percentage is calculated as the Gross Profit divided by the Contract Price.
Gross Profit is calculated as the Selling Price less Cost Basis less Selling Expenses.
Each of these elements can be revised in the year the contract is signed (before you file your tax return).
Additional Information: