increment-decrement-number.el

(defun change-number-at-point (op val)
  "Change number of at point by applying OP and VAL."
  (save-excursion
    (save-match-data
      (or (looking-at "[0123456789]")
          (error "No number at point"))
      (replace-match
       (number-to-string
        (mod (funcall (apply-partially op (string-to-number (match-string 0))) val) 10))))))
(defun increment-number-at-point (&optional arg)
  (interactive "p*")
  (change-number-at-point '+ (or arg 1)))
(defun decrement-number-at-point (&optional arg)
  (interactive "p*")
  (change-number-at-point '- (or arg 1)))