Each version of PC*MILER is a separate installation of the software. Your new version will be a new install rather than an upgrade. If you have an older version or versions of PC*MILER on that machine, they can remain or be removed. But, if removing, that should be done prior to installing the new version.
See instructions here if you have to uninstall an older version after already installing the new version.
If you are using any of the following software add-ons - Connect, Mapping, Spreadsheets, BatchPro, or TCPIP - they can only be used with one version of PC*MILER per machine. They will default to whatever version of PC*MILER was most recently installed.