How to write at a sppecific location

[Pooja]

Hi All

I have one csv file which has some data related to test results.

It has following information
TestId,Expectedres,Actualres
101,12,13
102,13
103,14

If I want to write ActualRes value in the file , How to do that.

I tried using seek but its not working. I am not able to write at a
specific location.

Please tell me what is the rght way. I have chked all the docs but was
not able to find any good solution.