Just in case it stops others wasting their time as I did ...
You can't !
You have to delete the account and set it up again.
Or you can just turn off the primary server (leaving it in position at the top of the SMTP page) and turn on the "secondary" server(s) you want to use.
Apple says: " If an SMTP server is being used as a Primary SMTP server by any account, you can't delete it without removing that account. Instead, disable the server.
Last Modified: Sep 2, 2014 "
Note: I had set up my accounts by syncing my 5s 8.1 with Outlook 2010.
You can't !
You have to delete the account and set it up again.
Or you can just turn off the primary server (leaving it in position at the top of the SMTP page) and turn on the "secondary" server(s) you want to use.
Apple says: " If an SMTP server is being used as a Primary SMTP server by any account, you can't delete it without removing that account. Instead, disable the server.
Last Modified: Sep 2, 2014 "
Note: I had set up my accounts by syncing my 5s 8.1 with Outlook 2010.