In the Unity Hub v3 Beta, click Open > Open Remote Project to see the list of your Plastic SCM repositories that contain a Unity project.
Note: In Unity Hub 2.4.4 and up, your Plastic SCM projects will be listed alongside Collaborate projects.
Select the project and click Next.
Select the Editor version and platform and click the change version button.
Your local Plastic SCM workspace will be created for you. The latest version of the project will be downloaded and the Editor will open with the latest version of your Unity project.