Before you do anything:
Backup!
Backup your files, backup the database.
(I.E. download all your files to your pc. Use PHPMyAdmin or whatever you have to administer your database and export the data to your pc)
(Normally you won't need it, but in case something goes wrong you can restore from that).
Then go to the forge and download cmsms 1.8.2. No diff packet here. Take the base packet which contains only English language or full packet with all languages).
Unpack to some place on your pc.
Upload the file overwriting the existing ones.
Make config.php writable (chmod 666)
In your browser go to
http://yoursite.com/install/upgrade.php
let the upgrade do its job.
When all is done:
remove the installation files from your server. Write protect your config.php (chmod 444).
Check if everything works.
Hope that helps
Cherry