How to change email address on iphone?

How to change email address on if there's one allready set up in Mail?

If you mean, how to change your mail address, then go to:
Settings > Mail, Contacts, Calendars
In the 'Accounts' section, go to the account you want to change, then go to:
Acount
Then, change you E-Mail address from there.

If you want to add a E-Mail address, then click on 'add account…' instead of going to your account.