I don’t know about changing your number online because I don’t have access to an online Metro by T-Mobile account, that is explained at the end of this answer. However, you can do it by calling the Metro by T-Mobile repair/support number from the device or another Metro by T-Mobile phone by dialing 611 or *611, properly identifying yourself by correctly answering a security question or two, possibly reading a security code from a text they send to your phone and tell them you want to change your number. If you can not properly answer the security questions, you will need to physically visit a corporate store (not a reseller) and have them help you.