Revised return AY 20-21

Yes you can

What If the earlier return has already been processed..?? Can I revise the same?

Yes it can be done

Yes revise the return before 31.5.21

Of course you cam 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.

Before 31.5.21

Yes, you ca revise your return for the FY 2019-20 even after receiving the intimation. 

Revise is by 31st May 2021.