assume , you are using a custom theme Vendor_yourtheme
Please copy
YOUR-MAGENTO-ROOT/vendor/magento/module-checkout/view/frontend/web/template/minicart/item/default.html
paste into
YOUR-MAGENTO-ROOT/app/design/frontend/Vendor/yourtheme/Magento_Checkout/web/template/minicart/item/default.html
now replace label "Qty" with "Quantity" as follows
<label class="label" data-bind="i18n: 'Quantity', attr: {
for: 'cart-item-'+item_id+'-qty'}">
</label>
Please empty your pub/static/frontend/ folder by running below command and flush magento cache
sudo rm -rf pub/static/frontend/*
Note: To display mini-cart "quantity" label properly, you may need to adjust some css class.
