(setf (symbol-function symbol) new-contents)

contents - If the symbol is globally defined as a macro or a special operator, an object of implementation-dependent nature and identity is returned. If the symbol is not globally defined as either a macro or a special operator, and if the symbol is fbound, a function object is returned.

new-contents - a function.

 (symbol-function 'car)  #<FUNCTION CAR>
 (symbol-function 'twice) is an error   ;because TWICE isn't defined.
 (defun twice (n) (* n 2))   TWICE
 (symbol-function 'twice)  #<FUNCTION TWICE>
 (list (twice 3)
       (funcall (function twice) 3)
       (funcall (symbol-function 'twice) 3))
 (6 6 6)
 (flet ((twice (x) (list x x)))
   (list (twice 3)
         (funcall (function twice) 3)
         (funcall (symbol-function 'twice) 3)))
 ((3 3) (3 3) 6)   
 (setf (symbol-function 'twice) #'(lambda (x) (list x x)))
 #<FUNCTION anonymous>
 (list (twice 3)
       (funcall (function twice) 3)
       (funcall (symbol-function 'twice) 3))
 ((3 3) (3 3) (3 3))
 (fboundp 'defun)  true
 (symbol-function 'defun)
 implementation-dependent
 (functionp (symbol-function 'defun))
 implementation-dependent
 (defun symbol-function-or-nil (symbol)
   (if (and (fboundp symbol) 
            (not (macro-function symbol))
            (not (special-operator-p symbol)))
       (symbol-function symbol)
       nil))   SYMBOL-FUNCTION-OR-NIL
 (symbol-function-or-nil 'car)  #<FUNCTION CAR>
 (symbol-function-or-nil 'defun)   NIL

Should signal undefined-function if symbol is not fbound and an attempt is made to read its definition. (No such error is signaled on an attempt to write its definition.)

setf may be used with symbol-function to replace a global function definition when the symbol's function definition does not represent a special operator.

(symbol-function symbol) ==(fdefinition symbol)