C# and Sql 2005 problem

I am unsure of what to do :( My conection with the DB is always open, it only close after exit button(exit aplication)
I writte this in button insert_DB_city:
conn="MY IP adres and path to DB on another computer";
SqlConnection connection1 = new SqlConnection(conn); // this is the problem

SqlCommand insertCommand = new SqlCommand(
"INSERT into Wood (city) values ("'+txtwood.text+")",
connection1);

connection1.Open(); // and this (but i know why, because conection is always open :) don't need that
insertCommand.ExecuteNonQuery();
connection1.Close();

My DB already have row city, and there is about 1000 articals in DB. I just need to insert information in colum city :(

I think i need to use SQL UPDATE Statement for my problem....