Are you referring to line 6b on Form 1040? Form 1040 line 6b is the taxable amount of your Social Security benefits. It is calculated based on your other income. The IRS must have changed some other income on your tax return that caused line 6b to change. What else did they change?
How did you fix line 6b? You can't just enter an amount on that line. TurboTax calculates line 6b based on other things that you entered.
Does the IRS letter actually say that you should amend your tax return, or does it just tell you to pay an additional amount of tax? Read the entire IRS letter carefully. If the IRS is correct, you might not have to do anything except to make the additional payment. Follow the instructions in the IRS letter.