I logged into my account and to my surprise I no longer see option to change my data plan,
looks like they made it a part of your voice plan, which means if you want to change it, you'll have to speak to an agent and it may affect your current contract, just like change to voice plans.
I have old grandfathered $25 data plan and wasn't gonna change it and loose it, but its kinda upsetting no longer to have flexible option to change to larger 3GB or 5GB with tethering and change it back to 3GB anytime you no longer need it. They must have eliminated it because people wanted to use tethering without jailbreaking but only sometimes without having to subscribe to 5GB data plan all the time every month.