(defun get-number-at-point ()
  (interactive)
  (skip-chars-backward "0123456789.-")
  (or (looking-at "[0123456789.-]+")
      (error "No number at point"))
  (string-to-number (match-string 0)))

(defun round-number-at-point-to-decimals (decimal-count)
  (interactive "NDecimal count: ")
  (let ((mult (expt 10 decimal-count)))
    (replace-match (number-to-string
              (/
               (fround
                (*
                 mult
                 (get-number-at-point)))
                mult)))))

(global-set-key (kbd "C-c r") 'round-number-at-point-to-decimals)