Enable PreOrder for All Products in a Collection

PreOrder Now doesn't currently support applying preorder settings directly to all products in a collection. We do have a workaround, though.

Use Tags to Enable Preorder for a Collection

Add a tag to all products in the collection you'd like to enable for preorder.
For example, add the tag summer2018 to every product in your Summer 2018 collection.
Go to the Products > Preorder Tags section in the app.
Enable preorder for products with that tag.

For a detailed guide, check this article on Product Tags.