How do i modify my year ending IRA balance?

[GeorgeM777]

You don't.  Your year ending IRA balance will be on your December IRA account statement and reflects the value of your IRA at the end of the year.  Assuming your question relates to determining the taxable and non-taxable amount of a distribution from your IRA, entering the year-end balance allows TurboTax to do a correct calculation of the taxable and non-taxable amounts of the traditional IRA distribution that was reported on Form 1099-R.  Without the entry of the year-end value, the calculation result will not be correct  because TurboTax assumes a zero year-end balance in the absence of the entry.  When you enter the correct year end balance, the resulting calculation will accurately reduce any erroneously high refund amount or any erroneously low balance due with your tax return.

[SweetieJean]

I think OP may be asking how to correct a data entry error for this.