If an earlier version of the Perforce QAC Eclipse is already installed on the machine, then the installation process will detect that it is already installed.
Click Next to start installing the update.
If, after upgrading the Perforce QAC Eclipse, you try to use a pre-existing workspace, the Perforce perspective and/or views might not be available (their content will be marked with an error). Because this issue is related to deprecated preferences in the existing workspace, you may wish to consider setting up a new workspace.
However, in the event that you wish to continue using the existing workspace, close all the old perspectives and views one by one, and then reopen them, using Window > Perspective > Open Perspective > Other... for the perspectives, and Window > Show View > Other... for the views.