You can delete a lead from the Inquiry Information page. There should be a delete button next to the save button.
You can reach the Inquiry Information page either by clicking on the lead if it is filtered as desired from the Leads page tab, or by clicking on the primary contact name while in the task page for that specific lead.
Please note: If you do not see a 'Delete' button you may want to check with your manager, as some partner organization or properties have restrictions set on their YGL account for property level YGL user to not be able to delete. If this is the case, you need a Corporate level YGL user to delete the lead on your behalf.