You can’t do that in the Online versions. So do not change anything or you will overwrite your 2018 return. Or are you using the Desktop installed program?
If you have the Desktop program you can do a What-If worksheet.
Go to Forms Mode, click Forms in the upper right or on the left for Mac. Then click Open Forms box in the top of the column on the left. Open the US listing of forms and towards the bottom find the What-if worksheet. It's right under Estimated Taxes.
Or start a new return to test in. In the Desktop program you can save your return with another name to test in.
