Q:

How to subtract 1 from the nth element of a list of numbers?

How do I subtract 1 from the second element of the list (3 6)? 
(setq x '(3 6))
(- (nth 1 x) 1)

The above quote does the math but does not return a modified list. 

A:

Using the built-in cl-lib package, you can write the following:
(eval-when-compile
  (require 'cl-lib))

(cl-decf (nth 1 x))

Alternatively, in vanilla Elisp:
(setf (nth 1 x) (1- (nth 1 x)))

Be careful, though, to avoid modifying quoted constant lists such as '(3 6) in-place, as doing so modifies the constant globally and may cause the code to behave differently in separate runs.
Instead, make sure to operate on a new list each time, e.g. via:
(setq x (list 3 6))

