You can't switch an existing account from POP to IMAP, or vice versa. You have to set it up again. Just add your account again as if it were a new account. '''File|New|Existing Mail Account'''. Are you sure your provider offers IMAP? If IMAP is available and advertised, Thunderbird should select it automatically. Otherwise you may have to enter the port numbers etc by hand. https://support.mozilla.org/en-US/kb/switch-pop-imap-account