You may have installed the previous version in the incorrect location.
After upgrading your Big Medium installation to a new version, if the update script or page footer tell you that you're still running the previous version, it's likely that you installed that previous version in the wrong location the last time around. This means that you have two versions of Big Medium on the server, and Big Medium is getting confused about which one to use.
Give this a try:
Your moxiebin directory should contain only .cgi files and two directories: Modules and plugins. If you see other directories (especially a directory named BigMed) or any .pm files (especially a file named BigMed.pm), delete those extra files and directories.
After doing that, re-upload these files from the new version of Big Medium to which you want to upgrade:
moxiebin/Modules/BigMed.pmmoxiebin/Modules/BigMed (directory and all files within)