You can still fill a revised return as the assessment there means assessment under 143(3) and not under 143(1). Have you tried filing a revised return?
Or else you can file a rectification request online against the assessment order you have received u/s 143(1) and you need to upload the XML file of your return. If assessment is completed under 143(3) which is not possible you can file a rectification request before ITO.
Since it's an exempt income there won't be an issue even if you skip it but it's always advisable to disclose all the income.
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