The following code can be pasted in your functions.php file and works immediate.

add_filter( 'woocommerce_checkout_fields' , 'puddinq_override_checkout_fields' );
 
// Hooked in function - $fields is passed in the filter!
function puddinq_override_checkout_fields( $fields ) {
 
    $fields['billing']['billing_address_1']['label']        = 'Street';
    $fields['billing']['billing_address_1']['placeholder']  = 'Street';
    $fields['billing']['billing_address_2']['label']        = 'Number';
    $fields['billing']['billing_address_2']['placeholder']  = 'Number';
    $fields['billing']['billing_address_2']['required']     = 1;
    $fields['shipping']['shipping_address_1']['label']      = 'Street';
    $fields['shipping']['shipping_address_1']['placeholder']= 'Street';
    $fields['shipping']['shipping_address_2']['label']      = 'Number';
    $fields['shipping']['shipping_address_2']['placeholder']= 'Number';
    $fields['shipping']['shipping_address_2']['required']   = 1;
 
    return $fields;
}

To unravel the code above: the filter holds the $fields in an array. We change the values of the array to get what we want. We are changing both the ‘billing’ and shipping form, changing the address_1 and address_2 field labels and placeholder. In the case of address_2 we also set the required to 1 to make the field required.

• All other fields remain the same
• The fields will hold the same place in mails, and orders.