There are two ways you can do this from your AppScenic Account.
1. Go to My Products and find the product you want to update.
Click on the title of the product and you will land on its page.
Now, go to the bottom of your product page and under Inventory, you have a field for Stock.
On the right side of the Stock field, you have a box called Infinity Stock.
Check that box and click Save.
2. Go to My Products and find the product you want to update.
Check the box on its left and choose Update Stock from the tabs on top of your products list.
Then click on Set Infinity Stock, confirm your updates and you have done it!
Now you know how to set a product as Infinity Stock.