We filed jointly last year but divorced in December 2019. Now I’m filing single. Where’s the section that deals with that? I’ve decided it might change the amount I owe

There is nothing to deal with that.  Don't know what you might be thinking of.  You don't mention that in your tax return.  You just file 2019 as Single or Head of Household if you qualify.

If you used Turbo Tax last year you should start a brand new account for yourself and not transfer from last year.  You can not remove a spouse or switch the order of names.

Online is only good for 1 return.  But you can use the same email address for 5 accounts.   How to start another return in the Online version  

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596

Divorced FAQ

https://ttlc.intuit.com/community/family/help/what-if-i-was-divorced/00/25830

Start a new account. Removing a spouse is not recommended

https://ttlc.intuit.com/community/family/help/how-do-i-remove-a-former-spouse-from-my-return/00/27202

Getting Divorced article

https://turbotax.intuit.com/tax-tips/marriage/getting-divorced/L20NC66cf

Video tips for Divorced couples

https://turbotax.intuit.com/tax-tools/tax-tips/Family/Video--Tax-Tips-for-Divorced-Couples/INF14631.html

And see what AGI to use

https://ttlc.intuit.com/community/agi/help/what-agi-should-i-enter-if-i-m-recently-divorced-or-widowed/00/26954