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Hi Its MamaTax Here!
You have got 12 months from 31 January after the end of the tax year to correct or amend it ie for your 31 January 2013 online tax return you have up until 31 January 2014 to amendment it without any issues with HMRC.
You just need to log into your Self Assessment online account, go to the 'at a glance' page and choose the option to amend your tax return.
Hope this helps.