It is difficult but possible to amend a return that was not originally prepare in TurboTax. Why are you not using the service that prepared your original return to amend it?
To amend a return in TurboTax not previously prepared in TurboTax, you will have to buy, download and install on your computer the 2017 tax program. Here is a link to get it: https://turbotax.intuit.com/personal-taxes/past-years-products/ Unless you need assistance to enter something specific, you can use any version because they all include the same forms.
You will have to start by recreating your original tax return in TurboTax to match exactly the return that was originally filed. Once you have created a matching return, you can then choose the option to amend it. Here is a link to help you with the amending process after you have recreated the original return: https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed