To delete an Airtable base, click the menu dropdown button on the base icon in the home screen, and select the "Delete" option. You will need to click a second red "Delete" button in order to confirm the deletion.
You can also delete a base from within the base itself by clicking on the dropdown menu arrow next to the title of the base, then clicking on the three dot menu on the right side of this modal to open up the base menu. From here you can click the "Delete base" option (which is shown in red): 
NOTE
If you delete a base by mistake it will temporarily be available to restore from your workspace trash for seven days.