Revision of return after return processed.

Yes you can. the last date is 31st May 2021.

Yes you can revise the return before 31.5.21

Of course you can go for revision. because intimation under sec143(1) doesn't amount to assessment.
one should file revised return before the expiry of time limit for revision or assessment by A.O. which ever is earlier.
here no assessment made and you have time limit till 31.05.21.
go for revision with out thinking.

Agree with ramaraju

With reference to sec 139(5), where it is clearly mentioned "return can be revised any time before the end of relevant assessment year or assessment, whichever is earlier".

Aren't it contradict it? Because I have also received the demand.

You received Demand notice because your IT Return has been processed, Processing of return is not an assessment order as per the act. 

Coming to end of relevant assessment year, govt has extended the date till 31.05.2021 from 31.03.2021. there is no contradiction.

Agreed