When a taxpayer amends their return they are fixing any mistakes or errors in the return, before these mistakes are discovered by the IRS.  Not all changes require an amendment, please see the IRS How To File An Amended Return With The IRS to determine if you will need to do so.  Also see Turbo Tax information below on whether or not to file an amendment.

https://ttlc.intuit.com/replies/3318732

If you have to do an amendment, you can do this in Turbo Tax by selecting the year you would like to amend in the link below.  Remember you have 3 years from the due date of the return or two years from the date you made a payment on your return to file an amendment.

https://ttlc.intuit.com/replies/3288565