HMRC only allow amended tax returns for previous years within a certain timescale.

For individual, partnership and trust returns this is within 12 months of the statutory filing date which is 31 January following the end of the tax year, or 3 months from the receipt of a Notice to File, whichever is the later.

For limited companies this is at any time up to 12 months after your filing deadline (known to HMRC as your 'filing date' or 'statutory filing date'). Your filing deadline is usually 12 months after the end of your Corporation Tax accounting period. So you can usually make an amendment to a return up to 24 months after the end of your accounting period. If you need to file later than the amendment window, you will have to submit the amended return in paper format but can use the printed version produced within the software.