Can not modify capital gains in state form

When working with federal or state returns (assuming you've already filed them), if you make an error, you must file an amended return. Most states show the standard tax form no. with an X suffix (e.g., you would use Form 1040X to amend a 1040 return). Also, most states do not allow you to e-file an amended return; you must snail-mail the paperwork to your state revenue office.

I have not filed my taxes yet.  Turbo Tax will not let me change the bad state data

Sorry, I assumed you had already filed. I only use TurboTax for my federal return but may still be able to help. In the field(s) where you have "bad state data,"hover your cursor to the left of the field entry. This should reveal a symbol that you can click to find the supporting data (where TurboTax got its [bad] calculation). Let me know if clicking this symbol helps to solve your problem.