You can change the amount for outstanding mortgage balance. Since the 1098 instructions ask lenders to report beginning of year mortgage balance, but the tax return limits based on average, it is acceptable to adjust this figure, and doing so will not cause any errors. As @KrisD15 mentions, there will be no additional scrutiny from the IRS for this permissible input.
Make sure you are using an allowed method to compute the average balance. You can use the average of first and last balance method, interest rate method, or statements provided by your lender.
