Deleting a theme via the WordPress dashboard is the simplest way to do it. If you have access to the admin screens, then this is the recommended way.

In your WordPress site, go to Appearance > Themes. You’ll see all of your installed themes displayed with the active one first in the list.

From the list, click on the theme you want to delete to view its details.

In the lower right-hand corner, there’s a Delete link. Click on that and WordPress will check you really want to delete the theme. Click the OK button in the popup and the theme will be deleted.