update-in.rkt

(define (update-in ht ks v)
  (cond [(empty? (cdr ks)) (hash-set ht (car ks) v)]
        [else (hash-set ht (car ks) (update-in (hash-ref ht (car ks)) (cdr ks) v))]))

(define test-hash (hash 'fruit (hash 'apple 'red 'banana 'green)
                        'veg   (hash 'aubergine 'purple)))

(update-in test-hash '(fruit banana) 'yellow)
;; => #hash((fruit . #hash((apple . red) (banana . yellow))) (veg . #hash((aubergine . purple))))