If windows form:
private void button1_Click(object sender, EventArgs e)
{
Messagebox.Show("Changes saved");
}
if webform:
<!DOCTYPE html>
<html>
<head>
<script>
function myFunction()
{
alert("Changes saved!");
}
</script>
</head>
<body>
<input type="button" onclick="myFunction()" value="Show alert box">
</body>
</html>