Problem to update existing app [missing key signing]

Hello,

I have been having some trouble to upload an update to my app that is already published in AppLab. I get an error that my update has been signed with a different key. That is in fact true. I have lost the key and had to create a new one to build my update. I acknowledge that I can't update an app with a different signed key and would probably have to upload the application as if it were new. However, how could I delete my existing app and upload it again? I can't find the option to unpublish my app.

Thanks