Mistakes can happen, but you should correct any mistakes as soon as you notice them on your tax declaration. You can correct them in a few quick steps:
Log in to your Self Assessment account
Go to ‘More Self Assessment details’ and then click ‘At a glance’
Click ‘Tax return options’
Choose the form you would like to correct
Make the corrections in the form, and submit the form again
If you filed the tax declaration on paper, you should print a new tax return form. Remember to make all necessary corrections. Once it’s done, send the new form to the HMRC via post.
You can make changes in your form within 72 hours after filing it. If you didn’t manage to correct the declaration on time, you should contact HMRC.
Take a look at the HMRC’s website to learn more.