Here is below link from which you can see how to add a banner from the admin:
Below is the step for the same:
1. Login to admin
2. Click on setting gear icon
3. Click on setting
4. Click on template preferences
5. click on advanced editor
6. Click on store template
7. click on home
8. Find banner URL which you need to change
Open an another window and click on setting gear icon and select file storage.
Upload banner you want to change.
You will get unique URL
Copy that URL and paste it to advance editor area.