The default collation of a collection may be specified during creation (https://docs.mongodb.com/manual/reference/method/db.createCollection/#db.createCollection). However, there is no way to change the default collation of a collection that already exists. For example, collMod does not allow changing the collation option.

This appears intentional:
https://docs.google.com/document/d/1SxbSBj50GurIDfUXwLBaEfy3h_SHNkJzia5luj87wWM/edit

Operations issued against the collection will respect the collection default collation, unless an overriding collation is set on the operation itself. Once the collection has been created, the collation cannot be changed.

However, implicit collection creation is more common than explicit. Not allowing default collation changes makes it difficult to discover collation needs later in the development/release cycle and forces large code changes to apply collation to cursors.