You have to sign up for an account online using the phone number assigned to your account, You can view the phone number assigned to you by going to settings, about tablet and then tapping on status. Once you sign up for an account online, you can then sign in and change the data plan.
Make sure the sim is in the tablet when you sign up for the account because the temporary pin will be sent to the tablet for verification in the status bar if on T-Mobile.
If you are using AT&T, go
HERE. T-Mobile
HERE.
It's not like an iPad where you can do this on the iPad.