How do I fix mistakes from last tax yr?

[DexterS]

If you need to make a change to a tax return that has already been filed, whether for this year or for a prior year you must file an Amended Return (1040-X)

You have to use the software appropriate for that year.

The online software for prior years is no longer available.  You may purchase the desktop software here:  https://turbotax.intuit.com/personal-taxes/past-years-products

Generally, to claim a refund, you must file Form 1040-X within 3 years after the date you filed your original return or within 2 years after the date you paid the tax, whichever is later.

When you get into the software, type "amend" in the search box at the top of your screen.

TurboTax will display a "Jump to Amend".  Click on that and TurboTax will walk you through the process.