Dear Dawit,
Apply a for loop on the datagridview pn the selected index changed of the drop down and then set the text box value like this:-
for(int i=0,i<dgv.rows.count;i++)>
{
if(e.RowIndex==i)
{
dgv.Rows[i]["TextBoxID"].Text=[value you want to set]
}
}
Mark this as your answer if it helps you out.
Thanks
Varun Sareen