We do our best to help here, but with a very common operation like this, there are also a ton of other resources available on the internet. If you need quick help and we aren't able to assist immediately, then Google would be an excellent place to turn.
To update a garrysmod install, you will need to follow very similar instructions to those at the bottom of our documentation on setting up a CS:S server at viewtopic.php?f=67&t=4107
1. Shut down your server.
2. Click "Start" and then click "Run".
3. Type "cmd" into the blank and hit enter.
4. Type these commands:
hldsupdatetool.exe -command update -game garrysmod -dir sourcebase
The hldsupdatetool will also show a help screen if you type "hldsupdatetool" alone into the command prompt window and press enter.