You need to have "Platform management: Users" permission to manage another user's credentials.
Deleting is irreversible, and we cannot restore permanently deleted items. Confirmation is always required before any deletion, so be sure you're not accidentally deleting the wrong item.
Delete a credential
Click your organization name on the lower-left corner of the page and select Credentials.
Select a credential type.
Click Show archived on the upper-left of the page to filter by archived credentials.
Click on the credential's right-hand side, and select Delete.

In the pop-up window, click Delete.
Bulk delete credentials
Click your organization name on the lower-left corner of the page and select Credentials.
Select a credential type.
Click Show archived on the upper-left of the page to filter by archived credentials.
Check the box next to each credential you want to delete.
Click Delete at the bottom of the page.
In the pop-up window, click Delete.
You need to archive a credential before you can delete it.