You can amend a tax return for up to 3 years from the original filing deadline to claim the education credit that you mistakenly did not claim. The American Opportunity Credit can be claimed for up to 4 tax years while you are an undergraduate student working toward a degree in the first 4 academic years.
You said in your post you claimed the American Opportunity and Lifetime Learning Tax Credit but you can only claim one of the credits in any tax year.
