How to change from POP to IMAP in El Capitan?

I went to setup my email account on my new MacBook.

It went so smoothly that it automatically set it up as a POP account and now it only receives the latest emails. It has not downloaded my previous hundreds of emails from the past.

How can I change from POP to IMAP so my previous emails will show up?

Thanks for any help

El Capitan ver10.11.6