This assumes you have done the following:
Rolling out an application is a way to replace an active edition with a new edition. In Editon Control Center, select the higher version of the application and select Rollout. You will be prompted to select a rollout strategy (atomic or grouped), a reset strategy (hard or soft), and a drainage interval (30 seconds by default).
A group rollout pushes the new edition to every application server in the cluster, whereas an atomic rollout pushes the new edition to each application server in the cluster, one at a time. Atomic means "one at a time". An atomic rollout guarantee's that all requests will be sent to the new edition of the application. If requests are submitted while the new application is being rolled out, the requests will be queued until the rollout is completed, and then the requests will be served by the new edition of the application.
Both a soft and hard reset will restart the application servers the application is being rolled out to. With a soft reset, process memory and any native libraries used by the application will not be refreshed. With a hard reset, process memory and any native libraries used by the application will be reset.
The drainage interval is the number of seconds to wait before the reset strategy is kicked off, to give time for HTTP sessions to complete.