Login at Sprint.com
Then to the right of the picture of your phone is 'manage my device'. Choose change phone #from that list.
To change your phone number on sprint.com:
o I need a different area code for this phone
o I am moving and need to change my billing address and phone number
Hope this helps.
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We've made some changes to the website. Try these instructions, Change your phone number, let us know if you have any additional questions!