Hi,
you can add this code in functions.php file located in the main root of your theme under wp-content/themes/<YOUR_THEME>
add_filter( 'yith_wcms_timeline_labels', 'yith_wcms_timeline_labels' );
function yith_wcms_timeline_labels( $labels ){
return array(
'next' => __( 'Next', 'yith-woocommerce-multi-step-checkout' ),
'prev' => __( 'Previous', 'yith-woocommerce-multi-step-checkout' ),
'login' => _x( 'Login', 'Checkout: user timeline', 'yith-woocommerce-multi-step-checkout' ),
'billing' => _x( 'Billing', 'Checkout: user timeline', 'yith-woocommerce-multi-step-checkout' ),
'shipping' => _x( 'Shipping', 'Checkout: user timeline', 'yith-woocommerce-multi-step-checkout' ),
'order' => _x( 'Order Info', 'Checkout: user timeline', 'yith-woocommerce-multi-step-checkout' ),
'payment' => _x( 'Payment Info', 'Checkout: user timeline', 'yith-woocommerce-multi-step-checkout' )
);
}
change the current label with the string that you wnat to display.
Have a nice day