If it works the same as with my carrier, here's what you need to do:
Go to Settings - Mail, Contacts, Calendars, select the email account you need to change. Now tap the email address:
This will lead to the following page:
To change IMAP settings, select "Advanced" to change SMTP settings, choose "SMTP".
In Advanced, you'll find this:
You'll have to scroll down to get there. Here you should be able to change the information.
In SMTP, this is what you'll find:
Either you'll add a server (bottom), or it's possible to edit the primary server:
Hope that helps.