Stripe Plugin: where is user data stored/how to modify?

The app I’ve built works well for brand new users, but I’m now trying to migrate existing users from our prior platform into the new one, and am facing a problem: to my surprise, I can’t find any way to set the stripe customer id of a user. It seems to be readable data from a user but I can’t even figure out where it is stored.

What am I missing here? There must be a way to do this…