Hi Shrawan the problem is not there with first block code but in the second block of code named update_Click even.
Please understand carefully.
When the TextBox doesnot contain any data then its value is String.Empty when you trying to convert this value i.e string.Empty to decimal using Convert.ToDecimal it throws an error so you need to change your code as I am suggesting for one TextBox please do for all other in following line.
SqlCommand cmd = new SqlCommand("UPDATE OrderEntry set Colour1Size1Qty1= '" + (float)Convert.ToDecimal(textBox5.Text) + "',Colour2Size1Qty1= '" + (float)Convert.ToDecimal(textBox34.Text) + "',Colour3Size1Qty1='" + (float)Convert.ToDecimal(textBox53.Text) + "',Colour4Size1Qty1='" + (float)Convert.ToDecimal(textBox72.Text) + "',Colour5Size1Qty1='" + (float)Convert.ToDecimal(textBox91.Text) + "',Colour6Size1Qty1='" + (float)Convert.ToDecimal(textBox110.Text) + "',Colour7Size1Qty1='" + (float)Convert.ToDecimal(textBox129.Text) + "',Colour8Size1Qty1='" + (float)Convert.ToDecimal(textBox148.Text) + "' where PIN='" + textBox208.Text + "'", con);
this is the target line which needs to changed as below.
(float)Convert.ToDecimal(textBox5.Text)
should be changed as follow.
(float)(!string.IsNullOrEmpty(textBox5.Text.Trim()) ? Convert.ToDecimal(textBox5.Text.Trim()) : default(decimal))
do for all the values....
Hope you got it..,
Thanks and Regards,
Rk_Hirpara