[CentOS] OT: Script Help

Sat May 18 19:15:36 UTC 2013
James Pifer <jep at obrien-pifer.com>

Sorry for the off topic, but don't a better resource. I'm not great at 
scripting, but need a quick script to modify a file.

I have a long file that has lines like this:

some text
some text2
CN=DATA.OU=XYZ.O=CO
some text3
some text4

And this repeats, but XYZ changes. "DATA" is always called data. (it's 
being renamed basically)

I need to change the middle line but leave the rest of the file as is 
like this:

some text
some text2
CN=XYZ_DATA.OU=XYZ.O=CO
some text3
some text4

Anyone know a quick way to do this? Any help is appreciated.

Thanks,
James