Add this code the functions.php file (CSS to hide update button) Source: https://rudrastyh.com/woocommerce/remove-update-cart-button.html

Update Cart Automatically on Quantity Change

.woocommerce button[name="update_cart"],
.woocommerce input[name="update_cart"] {
	display: none!important;
}

====================================================================


add_action( 'wp_footer', function() {
	
	?><script>
	jQuery( function( $ ) {
		let timeout;
		$('.woocommerce').on('change', 'input.qty', function(){
			if ( timeout !== undefined ) {
				clearTimeout( timeout );
			}
			timeout = setTimeout(function() {
				$("[name='update_cart']").trigger("click"); // trigger cart update
			}, 500 ); // 1000 for one second delay, half a second (500) seems comfortable too
		});
	} );
	</script><?php
	
} );