Manage the products

Try this in your custom css:

.in-stock {visibility:hidden}
.in-stock:before {content:"In stock"; visibility:visible}

If your theme does not have a setting where you can enter custom css, you can use a plugin like this one:
https://www.ads-software.com/plugins/simple-custom-css/

If it doesn’t work, your theme markup may be different. Please post the url to a product which is in stock.