[CentOS] sed a particular line in file

Fri Apr 9 10:20:03 UTC 2010
Tom Brown <tom at ng23.net>

I need to change a value in a file, but this line occurs more than once in
this file therefore as i know the line number this value appears on how can
i change that?

I thought i could use sed to change a particular line number but i dont see
that in the man page, i am trying to change a value from line number 6

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