Instructions
Connect to your GMod server and download the "HLDS Update Tool" from Valve's website (see Resources).
Double-click on the installer to start the installation process. Select a directory in which you would like to install the update tool and click "Next." Select your region from the drop-down menu and click "Next." The program will now install the HLDS update tool on your computer.
Click "Start" and select "Computer." Navigate to the folder where you chose to install the update tool and double-click on "HLDSUpdateTool.exe" to allow the tool to update itself.
Right-click anywhere on your server's Windows desktop and select "Create shortcut," then click "Browse." Select the "HLDSUpdateTool.exe" file in the file browser and click "Open." Click "Finish," then right-click on the shortcut you created and select "Properties." Type or cut-and-paste "-command update -game garrysmod -dir C:\srcds" in the "Target" field. Replace "C:\srcds" with the location of the folder where you installed your SRCDS files. Finally, click "OK." You can now double-click on the shortcut to install any available update to the SRCDS application.