Before deleting a Test Template there are a few things to keep in mind:

• Deleting a test template also deletes all test cases based on the template. Test run results in already-completed test runs based on the edited test templates are unaffected, but rerunning the test run will not include the deleted test cases, producing results that differ from previous runs.
• The Force.com multitenant architecture limits cascading deletes to 100,000 combined object records. Before deleting a test template or test case you suspect might have more than 100,000 associated records, first delete the associated test runs to avoid causing a situation that requires the assistance of Salesforce support. Refer to the Delete Operation Too Large Salesforce knowledge article for details.

To delete an existing test template: 

1. From the Test Templates page, initiate deletion by either:
   • Identifying the template to delete, clicking the dropdown arrow at the right end of the row, and selecting Delete.
     Or:
   • Selecting the checkbox of the test template to delete and clicking the Delete (trash can) icon.
2. Click Ok to delete the test template.