This is a feature that has been gone for some times now. Woocommerce intentionally removed the plus and minus signs. To know more regarding it please refer to this article: https://github.com/woothemes/woocommerce/issues/4643
As the article says: “In WooCommerce 2.3, the quantity increment buttons have been depreciated, as most browsers now support
<input type="“number“" />
If you want to keep them, install and activate this plugin: WooCommerce Quantity Increment plugin.