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:
Click the drop-down arrow next to the Unity Version to select a different version of the Editor. If you try to open a Project with an older version of the Editor than the one it was created in, Unity warns you that downgrading the Project might result in data loss and asks you to confirm your selection.
Click the drop-down arrow next to the Target Platform option to select a different platform.
In the Editor, click File > Open Project to open the Hub and access the list of available Projects.