First the "change" is probably interest paid for the delay in processing which sadly is taxable income on the 2021 return you file next year.
If it is the same letter I have seen it gives a "laundry list" of possible "errors" and in almost all cases this year the only one that applies to you is the last one for "math" errors. What they are referring to is line 30 of the 1040 for the Refund Recovery Credit. The IRS checked their records and are disagreeing with some or all of the amount of stimulus money you say you did not get in advance that is listed on line 30 .
So review your records ... did you enter the incorrect amount ? If so then you have to initiate a trace on the missing payments.
If that is not the problem then it is possible that reducing your unemployment income COULD have reduced the CTC and/or EIC especially if you did not use the 2019 earned income figure to compute the credits. These late tax law changes have been a headache for the IRS and since this had to be reviewed by a human and humans can make errors so this may be a processing error.
This PUBLIC forum is not the right place for you to get answers ... I suggest you contact TT support on Monday to see if they can give you personal assistance ... or the audit support number may be better...
Please use the following link to contact Customer Support for further assistance.
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
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.
https://support.turbotax.intuit.com/irs-notice/audit-support/
