Yes, you can correct a previous entry for installment sale expenses.  Here are the steps to get back to that section of your tax return:

• Sign into your account and click Take me to my return
• In Search (upper right of program) type installment sales and click the Jump to installment sales link from results
• Follow the TurboTax prompts to see what you have entered and make changes as needed

If you need additional assistance, please comment and I will assist further.