How do I go about fixing my 2018 tax return that was rejected for not having the right agi number I filed my return on my computer

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Find your 2017 form and retrieve the correct AGI figure.  Load up TT again and easy step through the filing process once more with the corrected number.  If you have your TT file from last year you may want to start up a second return and use that return as the base.  That will automatically transfer AGI.  You can copy anything that is different into the new file at the appropriate sections.