If you are unhappy with a decision about your benefits or tax credits you can ask for it to be looked at again (revision) or you can formally appeal against the decision to an independent tribunal. If you are successful, any change is normally backdated to the time of the original wrong decision. The rules for benefits and tax credits are slightly different.
There are strict time limits if you want to challenge a decision. If you miss the time limit, which is usually one month from the date of the decision letter, you can only put in a late challenge in exceptional circumstances.
For more information select the link entitled challenging decisions on the right side of the page.