simple_list = [1,2,3,4]

# append an element
simple_list.append(5) # now simple_list is [1,2,3,4,5]

# append all the elements of another list (NO NESTING)
second_list = [6,7,8]
simple_list.extend(second_list) # now simple_list is [1,2,3,4,5,6,7,8]

# replace an element by simply giving the index and the new element
simple_list[0] = 100 # now simple_list is [100,2,3,4,5,6,7,8]