If you mean “change workspace” go to File -> Switch Workspace
I took this question to mean how can you change the Default workspace so that when Eclipse boots up the workspace you want is automatically loaded:
Go under preferences then type “workspace” in the search box provided to filter the list. Alternatively you can go to General>Startup and Shutdown>Workspaces.
There you can set a flag to make Eclipse prompt you to select a workspace at startup by checking the “Prompt for workspace at startup” checkbox.
You can set the number of previous workspaces to remember also. Finally there is a list of recent workspaces. If you just remove all but the one you want Eclipse will automatically startup with that workspace.