In some Cases we may need to use qty to use decimal in our store so inorder to enable this we have two provisions to select that to each product based or we can totally declare to allow decimal for qty values.
Display settings for each individual product
Go to the following link : Admin panel > CATALOG > Products > edit any product > Quantity > Advanced Inventory
After That you can see->
After that Click save to Reflect that on the website.
For Changing this for all products >
Admin panel > CATALOG > Products > Select All or Select All on This Page and also can choose many different products
Select all products from the products page
After that click on update attributes->
Then Click save after modification.
