If you mean you cancelled service and were still charged, it would depend on when you cancelled. T-Mobile (as all the other carriers also) do not prorate months so if you cancel after your billing period starts you will be charged for the whole month even if you cancelled service 2 days after your billing period started.
If you downgraded service it would be similar to the above statement in that whatever was removed or downgraded wouldn't take effect until the next billing period begins.
If those don't answer your question the will need a little more of a description as to what the issue is.
Hi, rishi97! Thanks for posting. stevetjr's general description here is spot on... but if you still have questions we're happy to help! Can you let us know what type of plan change you made? Did you switch data features (upgrade or downgrade?), cancel a service, or change your entire plan? When in your billing period did you make the change -- as in, on what date, and what day does your bill cycle close?