I assume you mean you want to change your iCloud email address and not your iCloud ID (which looks like an email address). If so, you can't change your iCloud email address without creating a new iCloud account. You can, however, create a new @icloud.com email alias address to use within the existing account that will deliver email to your existing iCloud inbox. This explains how: http://help.apple.com/icloud/#/mm6b1a490a. Be aware that an email alias is permanenly linked to the account you create it in. It cannot later be used to create a new iCloud account, or be moved to another account.