if you are doing Prod / Test (Provided if you have huge amount of data in that desired datanode that you are going to decommision.
Increase the Datanode network bandwidh.
Maximum number of replication threads on a DataNode and
Hard limit on the number of replication threads on a DataNode
Click on the Host that has Datanode Role - Perform Decommision and once the decommissioned is finished
that you can monitor in the Namenode Web UI 50070 .
Sometime I would rather leave it as decommisioned state and later if you want to include the instance back as for scalability you follow the above same procedure instead of decommision u will click recommision that it joins back to the cluster .
if you want to delete the instance after decommisioning please follow the manual