Using checkbox

Hi
Your button "Alterar" is an HTML button. The onclick event (onclick="test();") you have written will try to execute only the javascript function called "test()". It will not fire up the code behind method test(). In order to fire up your code behind you need to use asp.net button control. Some thing like the following

<asp:Button id=Button1 Text="Click Me" onclick="Button1_Click" runat="server" />

and the code behind should be

void Button1_Click(object Source, EventArgs e)
       {
          Response.Write("You clicked the button");
       }

As your button is HTML button, you need to assign your event to onserverclick as

<input id="Button1" type="button" runat="server" value="Alterar" onserverclick="test"/>

and change your server side code as

protected void test(object sender, EventArgs e)
    {
        string msg = String.Empty;
        if (check1.Items[0].Selected)
        {
            msg = check1.Items[0].Text + "<br />";
        }
        Response.Write(msg);
    }

It'll work.

Change this:
public string test(){

to

function test(){

What you are doing is calling a client side JS function. You don't need to write public string thing. A normal JS method that can return anything.

onclick="teste();"

Spelling mistake - the method name should be test().