is it possible to decrease the size of root volume of EC2 ?

No, it is not possible to decrease the root volume of EC2. (you can only increase)

can we attach new volume to the EC2 or server ?

Yes, we can attach multiple volumes to an EC2 instance.

is it possible to detach the volume from the instance ?

Yes, it's possible to detach the additional volume not the root volume.

is it possible to detach the volume from the instance ?

can we attach new volume to the EC2 or server ?