Only the IRS can tell you what they changed on your return. What does the letter say? What is the CP or LT # at the top?
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 line 72. That happens when you are married and enter both spouse's W2s under the same name.
