Update a Specifice line of TextFile without causing Sharing Violation

Hi all,
I’m creating an Application in .net 2.0 which take data from a text File without causing sharing violation and Update a certain line after reading that data.

I Read file using Stream Reader but not able to find any thing by which i can goto specific line number to update Data.
e.g
A
B
XYZ
LMK
LMT

When i read that i want to change A to C.

I required your help in this regard
Thanks