(def thing 1)

; general form is 
; (alter-var-root var-to-be-altered 
;                 function-to-apply-to-old-value)
(alter-var-root #'thing inc) ; value of thing is now 2
; equivalently ...
(alter-var-root #'thing (fn [old-val] (inc old-val))) ; value of thing is now 3
; also equivalently ...
(alter-var-root #'thing #(inc %1)) ; value of thing is now 4