How can I remove my mother from my list of dependents for 2019?

If you did not file your tax return yet you can go to My Info to edit or delete a dependent.  if you already filed your 2019 return, you have to wait until it has been fully processed before you can prepare an amended return on a Form 1040X to remove a dependent.

How do I amend my 2019 TurboTax Online return?

If you are doing this just for the stimulus your parent may get I suggest doing nothing right now and simply don't claim them on the 2020 return  and they will get the credit on the 2020 return.  If the parent files a paper return now and you amend the return  the money will not come any faster and it causes less of a mess.