Manipulating Lists within lists in python

Use a list comprehension to get the new elements, and optionally slice-assign to replace the existing elements in the list.

new_aList = [list(reversed(x)) for x in aList]
aList[:] = [list(reversed(x)) for x in aList]

You could reverse each item like this

>>> aList = [
...     [1,2,3,4,3,2],
...     [2,3,4,5,4,3],
...     [2,1,2,3,4,3]
... ]
>>> aList[0].reverse()
>>> aList[1].reverse()
>>> aList[2].reverse()
>>> aList
[[2, 3, 4, 3, 2, 1], [3, 4, 5, 4, 3, 2], [3, 4, 3, 2, 1, 2]]

But in general it's better to use a loop since aList could have lots of items

>>> aList = [
...     [1,2,3,4,3,2],
...     [2,3,4,5,4,3],
...     [2,1,2,3,4,3]
... ]
>>> for item in aList:
...     item.reverse()
... 
>>> aList
[[2, 3, 4, 3, 2, 1], [3, 4, 5, 4, 3, 2], [3, 4, 3, 2, 1, 2]]

Both of those methods will modify aList in place, so the unmodified version is destroyed. Heres how you could create a new list and leave aList unchanged

>>> aList = [
...     [1,2,3,4,3,2],
...     [2,3,4,5,4,3],
...     [2,1,2,3,4,3]
... ]
>>> new_aList = []
>>> for item in aList:
...     new_aList.append(list(reversed(item)))
... 
>>> new_aList
[[2, 3, 4, 3, 2, 1], [3, 4, 5, 4, 3, 2], [3, 4, 3, 2, 1, 2]]

another way to reverse a list is to use this extended slice trick. The -1 means step through the list in steps of -1 ie. backwards.

>>> new_aList = []
>>> for item in aList:
...     new_aList.append(item[::-1])
... 
>>> new_aList
[[2, 3, 4, 3, 2, 1], [3, 4, 5, 4, 3, 2], [3, 4, 3, 2, 1, 2]]

Instead of explicitly making an empty list and appending to it, it is more usual to write a loop like this as a list comprehension

>>> new_aList = [item[::-1] for item in aList]
>>> new_aList
[[2, 3, 4, 3, 2, 1], [3, 4, 5, 4, 3, 2], [3, 4, 3, 2, 1, 2]]

bList = []
for l in aList:
  l.reverse()
  bList.append(l)

For more fine-grained manipulation of lists, the position of values can be swapped using tuple unpacking (which neatly avoids having to use temporary variables):

aList[0][1], aList[0][3] = aList[0][3], aList[0][1]

After this operation, the second and fourth values would change places, and so aList[0] would look like this:

[1, 4, 3, 2, 3, 2]