incorrect syntax near the key word 'UPDATE' [closed]

You have to use a parameter in the code and NEVER concatenate in the string

The structure should look like the following:

using (SqlConnection conn = new SqlConnection("<connection string>"))   
{   
    conn.Open();   

    string query = "UPDATE NombreTabla  SET campo1=@param1, campo2 = @param2 WHERE id = @paramid";   
    SqlCommand cmd = new SqlCommand(query, conn);   

    cmd.Parameters.AddWithValue("@param1", TextBox1.Text);   
    cmd.Parameters.AddWithValue("@param2", Convert.ToInt32(Textbox2.Text));  
    cmd.Parameters.AddWithValue("@paramid", Convert.ToInt32(Textbox3.Text));  

    cmd.ExecuteNonQuery();   

}

If the method you use cnn.update () does not let you assign parameters then you should change it, maybe define a List<SqlParameter> to be able to specify from the outside the parameters that you assign to the update