I thought the latest stimulus bil included tax changes for 2020 returns. If so what changed and how do I apply it to an already completed but not yet filed return?

There is a new unemployment exclusion that goes on 2020 returns.  Some of the other changes like child credits are for this year 2021.  

Did you get unemployment?  Up to 10,200 can be excluded.  Just enter your 1099G.

Enter a 1099G for unemployment under

Federal on left

Wages & Income at top

Unemployment - Click the Start or Update Button

If your modified AGI is $150,000 or more, you can’t exclude any unemployment compensation. The exclusion will be on Schedule 1 Line 8 as a negative number. Your unemployment compensation will be on Line 7 of Schedule 1. The result flows to Form 1040 Line 8.

If you have automatic updates enabled, 

open your tax return now in TurboTax and you will find that things are not as you remembered them.

Always make a paper copy of your completed return for your record and review it carefully before you file.