You have several options when opening an existing Project.
In the Unity Hub, click on the Project to open it with the assigned Unity Editor version. If the Hub can’t find a matching Editor version for the Project, it displays a warning message asking you to download that version.
You can also:
In the Editor, click File > Open Project to open the Hub and access the list of available Projects.
Note: For information on using Unity versions prior to 2019.1, see Starting Unity for the first time in the 2018.4 documentation.