Manish,
Agile Central Workspaces can be set to either Drag-and-drop ranking or Manual ranking, not both.
In order to change a Workspace ranking method, you must be a Subscription or Workspace Administrator. Changing a ranking method will impact all the users in the Workspace, so it is necessary to ensure all the users are willing to switch. Your rank method is administered separately for each workspace. Each method has advantages and disadvantages for your style of prioritization. Consequently, changing from one method to another requires foresight and planning. Drag-and-drop ranking is the default rank method for new workspaces.
I think you have already seen them, but just in case, here are the relative Help documents for ranking and changing the ranking method.
Drag and Drop ranking | CA Agile Central Help
Manual ranking | CA Agile Central Help
Michael