how to modify the csv file

[muttu2244 at yahoo.com]

hi everybody
am trying to modify a particular string from a csv file.

for ex

say i have

 compName,   ipAddr,   MacAddr,    Os

  sdfsdf   ,    129.122.12.34  , dsfiasdf, wsfdjhs
  hsdfdf  ,  123.234.34.34, dsfiasdfds, wewr

etc

now if i got to modify one particular line from these three rows , how
can i do that.

thanks in advance
yogi