Can I use my tax return to estimate next year’s taxes

If you are referring to 2019 estimated tax payments then see this TurboTax support FAQ - https://ttlc.intuit.com/community/tax-payments/help/can-turbotax-calculate-next-year-s-federal-estimated-taxes/01/26437

If you are referring to your tax liability or a tax refund for 2019 then use the TurboTax calculator TaxCaster - https://turbotax.intuit.com/tax-tools/calculators/taxcaster/

What-if worksheets are available but only if using the TurboTax desktop CD/Download editions.

You CANNOT massage the .tax file in any way when using the TTax On-Line version. ANY changes (what-ifs) that you make will PERMANENTLY change your .tax file. 

You have 3 options: 

Create a new On-Line account for each 'what-if' that you want to try. In TurboTax Online, you'll have to create a test account separate from your regular TurboTax account and create a test return there. We recommend adding "test" or similar to your User ID so you can easily distinguish it from your actual account.  

OR

Buy the Desktop version and install it on your computer --you can download the .tax2017 file from your On-Line return and use it to open your Desktop version. (You CANNOT upload the .tax2017 from a Desktop return to work in the 2017 On-Line version.)  The Desktop version CAN do 'What-If' & test return scenarios. 

OR

Use the estimated payment section in either program just do not change the filed return info ...

To access your current year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the bottom of the screen and Click on Add a state

This will take you back to the 2018 online tax return.  If you have filed the 2018 tax return do Not make any changes to the return that was filed.  Only access the estimated tax section and then print the estimated tax forms if needed.

1. With your tax return open, enter 1040-es in the Search box located in the upper right of the program screen.  Click on Jump to 1040-es
2. Answer No to the question,  Do you want to change your W-4 withholdings for 2019?
3. On the next screen, Review Your Estimates, select Review Now.
4. Proceed through the interview, answering questions about your 2019 filing status, income, deductions, etc.
5. Eventually, you'll come to the Print Vouchers screen; answer Yes and we'll include your 2019 1040-ES payment vouchers in the Print Center.