I think you might be a littel confused here.
It looks like n is used in your equation to represent the sample (or iteration) number.
So n is a positive integer number, starting at 0, and incrementing.
the small constant η is called the step size, or learning rate.
It is chosen.
It is a constant so when you choose it, it stays the same.
It has to be chosen to fit some constraints, not sure but I don't think this is hard.