I use this app to work out tax etc. for overtime at work, and when working out earnings stuff to do with my job, and noticed a few weeks ago they had a new feature for renewing tax credits. You can only use it once your renewal comes through.
Just got mine today and completed it in seconds. I don't have any change of circumstances that materially affect my claim so it was really easy & straight forward to do. Saves all that hanging on the phone!