I have Iphone and want to change Apple ID in it. How I can do it?

Do you want to change the email address that you use to access your Apple ID, or to change to a completely different Apple ID account?

Note that if you change to a different account, and not just change your email address for your Apple ID, you will have to repurchase or download all content bought with the previous ID.

Anyway, to change your email address for your account→Apple ID: Changing your Apple ID

In order to change Apple ID, you need to sign out from the existing Apple ID, using these instructions:--> Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV or Mac – Apple Support (UK)

Regards

Giulio