This customization is not available, you need to override the meta file and child-theme then replace the code that displays the SKU with Stock.
If you can’t do it yourself, I will charge you for this request, I have provided you with a lot of customizations for free before.
SKU stands for “stock keeping unit” and – as the name suggests – it is a number (usually eight alphanumeric digits) that retailers assign to products to keep track of stock internally, once it arrives from a warehouse or distributor.