Assume that the column is called "myColumn".
myDgv.Rows[0].Cells["myColumn"].Value = "hello";
Replace Rows[0] with the number of the row you are interested in.
[EDIT]
Dim str As String = InputBox("Enter new string")
Dim current = DataGridView1.Rows(0).Cells("DataGridViewTextBoxColumn1").Value.ToString()
DataGridView1.Rows(0).Cells("DataGridViewTextBoxColumn1").Value = current & str
I have broken it into two steps but you could do it all in one line.
BTW: Sorry for posting C# code instead of VB before. :)
[/EDIT]