There are a couple of ways to handle this. If you work through the estimated tax section of Turbo Tax, it will give you the opportunity to enter estimates of your 2019 income, and then it will adjust the 1040-ES vouchers accordingly. But keep in mind that as long as you pay in 100% of your 2018 actual tax liability, you will not owe any penalty to IRS no matter how much you are underpaid at the end of the year. So if you have the discipline to just set aside the additional taxes yourself in a money market account, you could make a little bit of interest by waiting to pay your balance until next year.
If you want to actually figure your 2019 taxes, just say "start a new return" when you open Turbo Tax, make up a name, and input whatever info you are estimating for 2019. Then you can see roughly what your tax will be next year and figure your estimates accordingly.
