How can I delete all emails on my iPad mini..

Your best bet might be to open Safari and log into your email account there and then select all and delete them. The specific steps depend on your email provider.

However this could also help:

https://support.apple.com/en-us/HT208661

I have this question too, and followed the link, but it says use Select All and my Inbox does not show Select All, just Mark All. The only thing that does is remove the "unread" blue dots.