1040 line 12 is the tax on your income. What is wrong with it? You change the tax by changing the income, deductions or adjustments in the lines above. Which you do by changing your entries. I don't think you can make overrides in the Online version, only in the Desktop program anyway.
Then it depends on what kind of income you have, if you have capital gains or qualified dividends the tax on line 12 is not taken from the tax table but is calculated separately from schedule D. The tax will be calculated on the Qualified Dividends and Capital Gain Tax Worksheet. It does not get filed with your return. In the online version you need to save your return as a pdf file and include all worksheets to see it.
For the Desktop version you can switch to Forms Mode and open the worksheet to see it. Click Forms in the upper right (upper left for Mac) and look through the list and open the Qualified Dividends and Capital Gain Tax Worksheet.
