Through WordPress Dashboard;
1. Go to Appearance>>Themes
2. Activate any default theme since you can’t delete your activated theme.
3. Now from installed themes open the theme you want to delete, you can find a delete button click that and your theme is succesfully deleted.
Through your FTP manager.
Using the FTP manager
1. log into your server and open the WordPress installation folder.
2. Go to wp-content/themes folder. Delete your theme folder.