Replacing the Glyphicons icons or Adding some new ones to it

I have bought this theme from themeforest , I already ask the developer for something this possible editing. The only thing they said is hire a expert to do this , They did not showed me a way to do it and thread closed , Looks like they want me to hire them for this job ,anyhow, I am going to rate there support pretty bad but I am actually wanting to make this change my self. Now when I looked up the theme files , It has a folder Named glyphicons , in /wp-content/plugins/nhpoptions/img/glyphicons , This folder has png images naming glyphicons_000_glass.png and so on , upto glyphicons_349_readability.png , I tought it would be too simple and I just need to replace the images with my own so I renamed my image glyphicons_349_readability.png and replaced the old one but this did not solved it. Still the same icon is appearing . Please tell me what am I missing , here is the screenshot of the dir