Changing Postman collection variables

I converted a Postman collection to k6 and now have a block containing the collection variables like this:

    collection: {
      var1: "foo",
      var2: "bar"

But I need to individualize the collection variables for different runs / different VUs.

Therefore I need to load test data from a json oder csv file and populate the collection variables.

How can I set them? And where should I do that? Within the default function? Or setup/initial?