You are able to prepare amended returns for 2014 and 2015 to claim the tax credit not claimed on your original returns.
You can find TurboTax software from previous years at https://turbotax.intuit.com/personal-taxes/past-years-products/.
The article below has specific amendment instructions for 2014 and 2015:
When you have prepared your amended return and are ready to mail Form 1040-X you should include the following forms:
- Form 1040-X
- The new (amended) Form 1040
- Any forms or schedules that were added or changed as a result of preparing the amendment
Please note that 1040-X cannot be e-filed - you will need to mail a paper return. It takes the IRS at least 2 - 3 months (and sometime up to 6 months) to process amended returns. Also, depending on whether the changes affect your numbers, you may also be required to file an amended return with your state.