I would like to first point out that this requires no mods and you can do this in the vanilla game. I also want to mention that this works with most Source Engine titles. (such as Team Fortress 2) Ok we went over this now you can learn how to change it below.

Open Half-Life 2 (obviously)
Go to "OPTIONS"
Go to "Keyboard"
Go to "Advanced"
Check the box that says "Enable developer console"
Click "OK"
Open the console by pressing ~
Type "viewmodel_fov 40 to 100" (without quotation marks and instead of typing 40 to 100 type a number between 40 to 100)
Exit the console by pressing ~ again or by clicking the X in the top right corner.
Load a game and it should work!