you can create your query as follows
string sqlQuery="update formtest set ";
if(!string.IsNullOrEmpty(Tbfname.Text))
{
sd.UpdateCommand.Parameters.AddWithValue("@firstname", Tbfname.Text);
sqlQuery = sqlQuery +" firstname=@firstname"
}
now use your logic to create your query with all textboxes. than in which textbox you enter data only which data will update rest will remain unupdated.