You can start an amendment and then cancel it to see the changes on your return. The links have instructions for each step. If you start the amendment to see the changes, be sure to undo it so that your filing records are correct.
If you have filed. The IRS will recalculate your return for the new unemployment exemption for you. There is no need for taxpayers to file an amended return unless the calculations make the taxpayer newly eligible for additional federal credits and deductions not already included on the original tax return.
https://www.irs.gov/newsroom/irs-to-recalculate-taxes-on-unemployment-benefits-refunds-to-start-in-may
You can log back into your return and it should automatically update your return and show you the new refund. But the updated return won't be sent. The IRS will adjust it for you.
In TurboTax Online, sign in to your account.
Select Tax Home in the left menu.
Scroll down to Your tax returns & documents.
Select 2020, and then select Add a State (you're not actually adding a state, this just gets you back into your return).