Unfortunately you can't, just keep both of them running.
It does look like this should be possible, as if they started to build this in but didn't get it finished. Hopefully they will get round to it at some point.
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It is not possible to install BIM Server 20 on top of your current BIM Server. This means it can be installed on the same hardware alongside with older versions, but it will be a separate application and has to be installed into a separate folder.