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I'd work on one problem at a time. Trying to do more will be confusing.

When you call a method that belongs to a class, you must have a reference to an instance of that class.
What data...

First quote says "I have...". The second asks about creating a method. I'm confused.

You need to look at the classes and where the data is that is to be updated and have the method located where...

The array: collection must be defined as an array of Product objects. collection[i] is must be a Product object.