If you don't wish to limit your product's online sales to available inventory in stock, you can just edit the product to disable Inventory Control, then click Next until you can Submit changes.
If you DO wish to limit sales to available inventory, edit the product and keep Inventory Control enabled, then click Next until you find the "Limit product sales to" field, where you would enter the quantity of units you have in stock, then click Next until you can Submit changes.