This answer applies only to Booksy Biz on web/tablet. For help with other Booksy products, contact the Support Team at info.gb@booksy.com.

Navigate to Inventory in Booksy Biz on web/tablet and open your Documents. Use the Stock Intake Confirmation to add units of stock of your products. You can add stock for multiple products via + Add item. You can only add stock of products you have first added to your inventory.