Can I delete "self-prepared" and insert paid preparer's name and firm information?

No. TurboTax is not intended for use by paid preparers.  

Intuit has tax preparation programs designed for professional tax preparers. Both ProSeries and Lacerte are tax-preparation software packages designed for professional tax preparers.

In addition to ProSeries and Lacerte, we also have ProConnect Tax Online, a cloud-based solution for the professional tax preparer.

If you prepare tax returns for a fee and want to use Intuit software, consider ProSeries, Lacerte, or ProConnect Tax Online, as the license agreement for TurboTax prohibits its use for commercial purposes.

For more information, please visit the Intuit Accountants website.

I am doing my Mom's tax return for free.  Is there a way to update the Preparer information?

No.  Turbo Tax is for self prepared returns.  You are just helping her.  One thing you can do if you have the Desktop installed program is enter your info as a THIRD PARTY DESIGNEE

My mother passed in Dec 2020 so I wanted to delete the self-prepared.  I have the Form 56 filed by her lawyer identifying me as the fiduciary and will gladly submit the 1310 if needed but "self-prepared" seemed inappropriate.  I saw a prior comment but I have already entered all her info and am ready to file.  Any recommendations? Thank you!

Since TurboTax is a do it yourself program, there is no way to remove this.  Having self-prepared on the return will not cause as issue.