To make a plan change online you must also be the account administrator/account holder, if you are and you do not have this option to to settings and passwords, edit online account and then scroll down to Accounts and then add account, follow the steps from there to link you online account with your billing account and then you will be able to set up a plan change, Please note when changing your plan online the new plan will not take affect until the start of a new billing cycle.
Hope this helps you BTW The steps Levi gave are correct!
Sign on to your account at sprint.com,
On the "OVERVIEW" page on the right select "I WANT TO" > "CHANGE PLAN OR ADD A PHONE"
It will open a new page with a pop up box, Select "CHANGE TO ANOTHER PLAN"
It will take yo to a page to select a plan compatible with your device.
Where in the world can you change your plan online. I found a faq that stated to just click next to your phone change plan/ add a phone, but there is no such option. Doesn't make any sense.