At first, you should uninstall the previous version (uninstall the mod, remove the mod's file), then install a new one.
If you see 500 error and cannot enter into the admin area, try to remove all the mod's files from /Sources dir, then remove "$sourcedir/Optimus/Class-Optimus.php" line from the value of variable integrate_pre_include (smf_settings table in database), and "Optimus::hooks" line from the value of variable integrate_pre_load.