/* SQL */

$sql = "UPDATE Exchange SET poundRate= $_pounds, dollarRate= $_dollars, euroRate= $_euros";
$result = mysql_query($sql) or die(mysql_error());

/* some other general usage */

$num_rows = mysql_num_rows($result);

while($row = mysql_fetch_assoc($result)) {
   echo $row['field1'].' '.$row['field2']; //etc...
}
========== IMPORTANT ========================= BUT, BUT Y U STILL USE THIS OLD GRUMPY THING??===================
PLEASE DON'T, FOLLOW UP WITH YOUR TOOLS, LEARN PDO and PREPARED STATEMENTS:
like... now!

/* Connect with PDO */

$connection = new PDO('mysql:host=localhost;dbname=my_db;charset=UTF-8','username', 'password');

$connection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$connection->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
// connection is all setup