Within online TT Premium software, when filling out a minor/dependents tax return subject to filing form 8615, I need help finding where to modify the parents tax rate?

While you are in the program:

1.  Click  Search in the upper right corner of the screen.
2.  Search the term "Kiddie Tax"
3. Click Jump To kiddie tax.
4. You will see the screen "Child's Unearned Income." 

Continue through that section, and after you see the parents' information, you will reach Parent's Taxable Income, and you can change the amount that originally entered, and make additional adjustments.