How do I amend an already amended return in TurboTax 2013?

I have just amended my 2012 return that corrected the home office square footage and depreciation schedule. I have not sent this return. I need to correct the updated information and depreciation on my 2013 and 2014 returns. Both these returns were done and e-filed with Turbo Tax, but my 2013 return has already been amended once. How do I amend the amended return in Turbo Tax?