You need to determine if the IRS calculation of tax owed is correct by preparing an amended return for 2014. If the amended return agrees, then just mail the IRS a check for the amount owed. You do not need to mail the amended return, the IRS will adjust your file for you. If you disagree with the amount owed, mail the amended return to the address in the CP22 (not the usual amending address) with a check for the amount you believe you owe, and a letter of explanation.
You will need a copy of Turbotax 2014 installed on your own computer. If you are a PC user, you can download a free amend-only version by following the amending instructions below.
You also need your original 2014 tax data file. If you don't have a copy, you can download it from your online account. If you can't find it, you may have more than one account and are looking in the wrong one. If you find it and it's locked, then you need to have used a paid version of Turbotax to file your 2015 tax return, in order to maintain full access to past returns (think of it like an annual subscription). If you did use a paid version but your 2013 file is still locked, it may be that you used a different account, and unlocks don't cross-transfer between accounts. You will need to call customer support.
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Customer support hours are given below. To contact support, use this web form and type your question or a keyword. You will get an appropriate phone number for your issue. (You will not get a phone number if you use the link outside of operating hours.)
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Amend https://ttlc.intuit.com/replies/3288565
