This is a very, very simple error to fix on your own because the error tells you
exactly what the problem is. I do not mean to sound rude, but it would be much faster for you to fix it than it takes to post this question and wait for a response.
Secondly, use a parameterized query. The way you have your code now, I could hack your db very easily. You have very unsafe code.
Something like:
SqlCommand cmd2 = new SqlCommand("update stkdetails set customer= customer+ @customer where empname=@empName and..., con2);
cmd2.Parameters.AddWithValue("@customer", row.Cells[7].Value);
cmd2.Parameters.AddWithValue("@empName", rows.Cells[2].Value);
...
// you finish the rest. Very, very simple.