During a standard installation of FAB Subtitler the previous version of FAB Subtitler will be uninstalled automatically. To be able to use the previous version of FAB Subtitler simply make a copy of the folder
C:\Program Files(x86)\FAB Subtitler
with all files, for example to
C:\Program Files(x86)\FAB Subtitler Version x.xx
and then start the file sttwin.exe from that folder to use that particular version of the software.