Hi,

If I understand correctly you wish to delete a legacy reCAPTCHA key from the legacy admin page.

This is done by selecting the key from the dropdown, then clicking the Settings (gear) button at the top of the page, then clicking the Delete (trash can) button at the top of the page. 

I am not seeing this drop down or settings gear. I think I  am in the new admin and  I see all the keys but cannot select them.  It only allows me to upgrade them 

Thank you for sharing more information! You are looking at Classic keys in the Cloud Console. You have two options at the moment (this is likely to change in the coming months as all reCAPTCHA keys will be migrated to Cloud Projects)

1. Use the legacy admin page ( https://www.google.com/recaptcha/admin/ ) to delete a key as described in my earlier comment 
2. In the Cloud Console, click Upgrade key to migrate a key to the current project. Once this is done you will be able to delete the key using one of the methods described here: https://cloud.google.com/recaptcha/docs/manage-keys