You can only change it when you file your 2020 return. And you can't file until Jan27. You enter or change it when you go through the File Tab.
For the Stimulus......We don't know if the IRS will have a way to change it Online before you file your 2020 return.
If you already filed it is too late to change it. If the bank rejects the deposit the IRS will mail a check to the address on your return. If you have not filed or it rejects you can change it by going though the FILE tab again.