In MongoDB how do you use $set to update a nested value/embedded document?

In MongoDB how do you use $set to update a nested value?

For example, consider a collection people with the following document:

{
  _id: ObjectId("5a7e395e20a31e44e0e7e284"),
  name: "foo",
  address: { street: "123", town: "bar" }
}

How do I update the street field embedded in the address document from "123" to "Main Street"?