Hello, 

You can use the replacement on allocated resource on the project. 

Select in the allocation section the outgoing resource
click on the "replace allocated resource" icon  

Fill in the new resource and the date on which it will replace the resource
Validate

All the planned load for the outgoing resource will be assigned to the new resource from the date entered

Thanks Arkhanje.
Would I need to mark closed in description TAB in resource after did steps you advised?

Yes,
closing the resource will hide it from all lists.

I tried to close the resource after replacement activity, but it still showing validation message.
How can it be resolved?

The resource is still responsible for 20 actrivities and 1 milestones that are not closed.
You must either close these items of change responsible.

But it is a question that, when I replaced resource allocation to existing resource from resource outgoing then why it is showing in those activities? What i will have to do for that? It is still showing outgoing resource even after i have replaced it with other resource!

The "replacement" process does not take into account replacement of responsible.
This could be an improvement, but is not possible yet.

I am also not being able to close resource with availability of that out going resource in assignment. specifically when assigned hours or real work is there in assignments for outgoing resource. Can you please help me to find a way so I can be able to mark close in resources for that outgoing resource?

You cannot close it if it has some left work.
You can use Resource Planning to identify these activities.

even after doing leftwork zero in assignment & marked closed in assignment for outgoing resource, it is not permitting to close resource from resources! It's showing same message what I have shared in this stream before.

You must change the responsible : a resource you want to close cannot be responsible for an non closed activity.

Thanks a lot. I changed resource in responsible & I has been done as per expectation. Appreciating you and your continuous support.