go to Dashboard > Appearance > Theme Editor > Theme Functions (functions.php)
past this code at the end of this file
Singal product Page :-
add_filter( 'woocommerce_product_single_add_to_cart_text', 'woocommerce_add_to_cart_button_text_single' ); function woocommerce_add_to_cart_button_text_single() {return __( 'Buy Now', 'woocommerce' ); }
Shop page
add_filter( 'woocommerce_product_add_to_cart_text', 'woocommerce_add_to_cart_button_text_archives' ); function woocommerce_add_to_cart_button_text_archives() {return __( 'Buy Now', 'woocommerce' );}