updating a .cfg file

Hi all

Just wondering how I would update a config file in order to change some of its values. This is just a sample config to show you want I mean

// graphics

Screen_Width=1024
Screen_Height=768
BPP=32

If I wanted to change the BPP for example via a GUI, what would I need to do programming wise to change the value of the BPP field from 32 to say 64? I can design the GUI I would need but seem to be stumbling on how to make such a change via code.

Thanks and love this site!