Changing Lines According To User Input In Text File

I have lines in my text file like this:
x y
x Y
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y

Lets say I have 20 lines like this. I want to change this lines according to user input. So user will input how many lines they want to change in terms of row and column. In my program
Row=TextBox5.Text column=TextBox6.text
User must enter initial value for x and y. Let say the initial value is
x=1 y=1.
Row=2 column=5..

Now my lines should look like this.
1 1
2 1
3 1
4 1
5 1
1 2
2 2
3 2
4 2
5 2
x y
x y
x y
x y
x y
x y
x y
x y
x y
x y

Since I enter row=2 only two set of lines is change other lines remain unchange. But in my coding all the lines is changing like this:
1 1
2 1
3 1
4 1
5 1
1 2
2 2
3 2
4 2
5 2
1 3
2 3
3 3
4 3
5 3
1 4
2 4
3 4
4 4
5 5
which is wrong.

This is my code
Dim lines() As String = IO.File.ReadAllLines("C:\wirematches.txt")
Dim xValue As Decimal = Val(TextBox1.Text)
Dim yValue As Decimal = Val(TextBox2.Text)
Dim altValue As Decimal = 2.54
Dim lineTracker As Integer = 0
[Code] .....
This code working fine for changes the column. But i dont know where i should write the code for row.