If the option does not exist to change your number online it's likely because you are not set up as an account administrator. When you setup a My Sprint account you can create the account with just the phone number or, you can put in your account PIN to authorize you as an account admin.
There should be a section on the My Preferences page that asks you to input your account PIN to gain full access to your account. Once this is done you can then change your number online.
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