This video shows an outdated UI. To see what's new visit https://youtu.be/7UxJzBGc-DI
In the updated builder, you can locate your data editor by clicking on the button as shown below:
Once you're in the data editor, you should be able to see your sheets and tables on the left hand corner.
There are two possible sources of data for your app. You will be prompted to use either a Google Sheet or Glide Tables.
In the data editor, you'll notice that Google Sheets are marked with a Google Sheet icon and Glide Tables are marked with a Glide Table icon.
Changing Your Source Sheet
If you are using a Google Sheet as your Data Source, you can change this in Glide. To do this, head over to the settings section and click on "Data". From there, click on the three-dot menu beside your Sheet's name and choose "Replace".
Replacing your app's source sheet
A Pop-up should appear, allowing you to change your source sheet for your app.
The (+) icon located on the left hand corner only allows you to add Glide Tables. If your data source is a Google Sheet and you would like to add more sheets, you can update your Google Sheet file and sync the changes to reflect any new data.