Here is a tool created by uhelios and Pix3lDemonand that let you allow to change carrier logo on iPhone/iPad, of your choice without jailbreak of iDevice. This tool simply creates a carrier configuration file which you have to load into your iDevice by using iTunes to change carrier logo. So, just follow the steps written below to do so.
Note: This guide just works for Mac OS X.
How to Change iPhone Carrier Logo
Download CarrierEditor for Mac and double click on it to mount
Open CarrierEditor.app and click “Let’s Get Started”
On your iDevice, tap on “Settings” > “General” > “About” and scroll down to “Carrier”, and just enter its number on the CarrierEditor and go next
Select your “Device” and “Carrier” from the available lists on CarrierEditor and go next
On Customization screen, you will find two boxes, one for black carrier logos, and the other is for etched logos. So, you have to drag and drop both logo types to create custom carrier logo into their respective place there and click on “Compile Carrier Update (.ipcc)” button
It will create two .ipcc files on your Mac desktop, for custom logo and for the original carrier logo
Note: You can select custom carrier logos from Zeppelin carrier logos folder which is bundled with CarrierEditor.
Launch iTunes and attach your iDevice to Mac with USB cable. Select your iDevice within the iTunes, under devices
Press and hold “Alt / Option” key, and click on “Restore iPhone” button in iTunes, then browse and select .ipcc file created for custom logo from desktop
Note: Do not select .ipcc file created for the original carrier logo.
When iTunes complets the carrier logo replacing process, just reboot your iDevice and after that you will find your custom carrier logo on the left top corner of your iDevice
Note: In case you want to get back the original iPhone carrier logo on screen, then follow the step-6 and browse for the .ipcc file created for original logo.