Whether or not you need to amend depends on how you filed last year. The 1099-G is for your overpayment of state tax in 2015 that you received in 2016.
But check it out in Turbo Tax just to be safe.
Print out your original return (if you haven't already) to keep for your records.
If you've already filed your 2016 return, go to Manage your return, and choose Amend (change a filed and accepted return). Follow the on-screen instructions.
Then go to Income>Other Income>State and Local taxes on Form 1099-G. Enter your 1099-G, and then look at the tax. If there's an amount in the upper left-hand corner, then that's what you owe. Turbo Tax will walk you through the rest of the steps to file your amended return.
If it's zero, then you don't owe any extra, and should not mail anything in. Put the 1099-G in your records for this year, and that's it!
You can find additional instructions on how to amend in the TurboTax FAQ below:
https://ttlc.intuit.com/replies/3288565
