You cannot permanently delete a Project (i.e. removing it from the "total" number of Projects available to your account).
However, you can reset a Project (completely clear it) by going to Project > Bulk Delete from within the Project you wish to reset. Copy and paste the reset_project option into the action window.
Please note that this is a permanent action.
Once the Project reset is complete, you may rename the Project that you have just reset by going to Project > Project Settings and inputting a desired new Project name, and saving.