Go to the Tests option within the Tests section. Click on the active test you want to delete (active tests are displayed in a blue rectangle). 

If you scroll down to the bottom of the page, you will see the option to delete your test. 

Heads up: the deletion cannot be undone afterwards so make sure you have the important details saved somewhere. You might want to use the same meta description for example in another test so we recommend to copy all useful information before you press delete.