>Our where i can change this code
you don’t

>it possible to start with a new order number every day..
use the following filter as you want to

add_filter( 'wppizza_custom_transaction_id', 'myprefix_custom_transaction_id', 10, 2);
function myprefix_custom_transaction_id($transactionId, $orderId){
/*
do whatever you need to do with the $transactionId
*/
return $transactionId;
}