I checked box to elect out of Qualified Economic Stimulus Property and I want to change this. How to I change this so I can use the special depreciation Allowance

go back to area of walkthrough where you checked the box. Then hit back button until you are asked to check the box. This time de-select it.

I also need to check "Yes" rather than "No" for the Economic Stimulus - Qualified Property box. I wasn't given the option to choose either box during the step-by-step instructions for entering fixed assets, but instead happened to notice there were errors on the Asset Entry Worksheets for each asset, telling me I needed to make a selection for that line. I mistakenly checked the "No" box while correcting the errors, and now that I know I answered incorrectly, I'm unable to select the "Yes" box. Going back to the step-by-step, I see no page where I can fix this. Is there anything I can do, other than deleting and re-entering all of those assets?

Are you using Turbo Tax Online or the software version downloaded to your computer?  If you are using the software, you may be able to go to the form itself and make the selection on the asset worksheets.