Update the majority of a collection

Given a collection of n documents, what is the fastest way to update one attribute in each of those documents with a different value?
Detail: I have two collections. 1 is users, and 2 is user info. User info is refreshed daily (short term fix) and as a result of the refresh, there are new ids for each document in user info. Each user document has the ids of every relevant user info document. So when I refresh user info, I now need to update a single attribute in every document in the users collection. Is there a better way of doing this than using a find one and update in a for loop?