How do I change value of selected cell on gridview

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C#

How do I change the value of a selected cell using a button for e.g. changing a cell value from 1 to 2

What I have tried:

int row = 0;
row = dataGridView1.Rows.Count - 2;
dataGridView1["Amount", row].Value = Keytxt.Text;
Keytxt.Clear();

This is my current solution but it only changes the value of the last cell in the column.