What is the details? What did the IRS change? Usually that is user error by entering something in the wrong place or leaving off some income.
Here's some ideas.....
I would check over your return and see if you can see anything wrong especially in the payments section 1040 Schedule 5. Make sure you agree with all the amounts listed. Like is the difference the same or close to the refund you got last year? Maybe you put down that you applied last year's refund to this year? Or you put down you made a payment with an extension or paid an estimated payment that you didn't make?
A common error is showing an estimated payment 1040 schedule 5 line 66 you didn't make or payment with an extension (line 71) you didn't send in. Sometimes people start an extension form on purpose or accidentally and an amount gets entered for a tax due to send in with the extension.
Or you claimed Excess Social Security paid on Schedule 5 line 72. That happens when you are married and enter both spouse's W2s under the same name.
Or if you got any 1099Misc forms you didn't report it as self employment income on Schedule C and pay the SE tax on it. It might have showed up on 1040 Schedule 1 line 21 for Other Income instead of line 12 business income.
Or the IRS changed your income. A common thing they change is the taxable amount of any 1099R for pension, 401K or IRA distributions or conversions on 1040. And if you get Social Security and they increase some other income it will make more of your SS taxable.
TurboTax does not receive any information from the IRS or your state after your return was filed, so no one at TurboTax knows about a letter you received from the IRS or the state.
(what the letters from the IRS mean)
I GOT A LETTER FROM THE IRS
I AM BEING AUDITED
If you purchased Audit Defense:
TurboTax has a special phone number for help with IRS/state tax letters, which will be easier than going through regular Customer Support. At the page below, click on the blue button that says "Get Help from TurboTax Support". Then it will ask you what tax year is your letter. Then it will ask you what the letter concerns. Then it will show you the phone number during posted business hours.
The IRS refers to as a "calculation error" any disagreement between your tax return and the calculations that they do based on information returns sent to the IRS directly from payers. Because of this, a "calculation error" indicated by the IRS rarely indicates a problem with TurboTax and most often means that the user of TurboTax either omitted some item of income or entered some item of income or a deduction or credit incorrectly.
As the others have said, if you describe the nature of the error identified by the IRS, it's likely that it will become apparent how the error occurred.
Please be clear about whether it was actually the IRS (i.e. Feds) or the State which sent the letter...and if the state..which state?