To index lists using lens, use ix. Example:
>>> let myList = [1,4,2,212,5]
>>> myList ^? ix 2 -- (^?) gets the result as a Maybe
Just 2
>>> preview (ix 10) myList -- preview is the non-operator version of (^?)
Nothing
>>> myList & ix 3 .~ 4 -- Set the 4zh element to 4.
[1,4,2,4,5]
>>> myList & ix 10 .~ 5 -- Inserting new elements is not possible
[1,4,2,212,5]
There is also another question about the Difference between at and ix.