how do you change a Pop email account to a IMAP

One of our family's email accounts does not work properly. We have multiple Apple products. My son's email was originally set up on my MacBook Pro as a Pop account. Then it was set up on my iphone as a Pop account. Next it was set up on the Ipad as an IMAP account because that was how my daughter's email was set up on it. The email works on the MacBook every once in awhile. It works on my iphone whenever a new email is incoming. The rest of the time it says invalid password. AT&T says that MacBook Pros can't handle Pop email. (although it used to work just fine) How can I change his email from a Pop to an IMAP?