I am missing income on my 1099-NEC form from a client. How can I enter estimated income instead of what's showing on the incorrect 1099-NEC form?

If your records show that you received more income than was reported on your Form 1099-NEC, then enter the additional income not shown on the Form 1099-NEC as 'other income' or 'general income'.  Use whichever category allows input for 'cash' income not reported on a tax form.

That way, you have reported all of your income accurately regardless of whether the Form 1099-NEC is corrected.