You cannot change the data source that an app is associated with. So it isn't possible to swap between Airtable and Google sheets, or to switch from one Airtable base to another.
If you need to make a change like this you will want to create an additional app.
Once you have done this to clear up the app you aren't using anymore you can delete.
From Setup Home go to App Settings and expand the Delete Your App section.
Access Setup Home by clicking the cog on the top right on the navigation menu, next to the edit layout.
Scroll down to the end of the page until you get to the section that say "Settings".
In App Settings, click on "Configure"
Press "expand" in the "Delete your app" column.
Press the red Delete button and follow the instructions that come up.
Deleting an app is irreversible, so you'll have to type the URL of your app into the dialog box to confirm you really want to go ahead.
Once you have done this select I understand. Delete my app.
You will then have an opportunity to make a new app.