You can make a change to your tax return after you’ve filed it, for example because you made a mistake. You’ll need to make your changes by:
31 January 2018 for the 2015 to 2016 tax year
31 January 2019 for the 2016 to 2017 tax year
31 January 2020 for the 2017 to 2018 tax year
31 January 2021 for the 2018 to 2019 tax year
If you miss the deadline or if you need to make a change to your return for any other tax year you’ll need to write to HMRC.
Your bill will be updated based on what you report. You may have to pay more tax or be able to claim a refund.