This is the summary of steps to update your PD Howler 11 to 11.4:

Open 'Steam'
Click 'Library', to find your Software products, including PD Howler 11
Right-Click 'PD Howler 11'
Click 'Properties'
Click the 'Local Files' tab
Click 'Browse Local Files'
Double Click 'Update' to open that new subfolder
Double Click 'Howler_11_4_RC131_SSE2_For_Steam.exe' to run and complete the installer.

> IMPORTANT: Pay attention to the exact path where you'll install it, especially if not using the default location. See details below.

For detailed step-by-step installation, and special notes for installations in different folder or on a different drive than the default location, please read this:

http://www.thebest3d.com/howler/steam/11.4-update/index.html