Can you provide some more detail? For example, I'm guessing that you want a script that prompts the user for the new value, and then inserts that value in the file.
Will you find the right place in the file based on the actual data, or on the file position (or location on a line)?
Are you familiar with basic BASH** scripting?---If not, start here: http://tldp.org/LDP/Bash-Beginners-G...tml/index.html
**Are you in fact using Linux? If so, then BASH is the most common scripting language