If, after making a successful submission, you need to make changes and file an amended return, follow these steps:
Successful response from HMRC at Date Time
The process will run the same as when you filed your original return.
You can currently file up to nine amended returns online but all amendments must be made within one year of the submissions deadline. This means that for the 2014–15 tax return, amendments must be made by 31 January 2017.
You can find further information in our knowledge base article What's the deadline for my self-assessment tax return?
Note: HMRC will not accept amended returns online for the 2012–13 tax year and any prior years. If you need to amend returns for these years, you will have to send a paper copy with a covering letter.