Editing Font Awesome PlugIn Icons

I’ve tried to change the Font Awesome icon on my page. The template code is this:


I would like to change it to:


But when I do this the screen is completely blank. I’ve just used “glass” here as an example. Can anyone help me sort through what I’m sure is a simple issue?

When in doubt, I Google. I could however find no reference anywhere for “flash_on” so I’d guess it may not be an icon at all that you can swap out. Could we perhaps see the line of code?