Editing external files

I am running Matlab in a unix enviroment and in my Matlab program I am calling an outside program (mav) to run with a unix command in Matlab:

unix('mav -u 5 -c 1 -s')

My goal is to edit the input files (three different files) for that external program and then run that external program again. I need to change one or maybe up to three values in the input files, and they are among various words and other data. How do I edit one value in a complex file? Call out the position of it perhaps?

The files are structured sort of like:

large paragraph words
words words words words words 
words
 
valuedescription: value
othervaluedescription: othervalue
 
more words
numbers

Whatever I do it needs to be from the command line so i can use the Matlab unix interface:

unix('command')

The file names are massMean.dat and propMean.dat, and they are in a different folder than the Matlab program and the 'mav' program I am running.