Updating defcustom variables in elisp code

[uzibalqa]

Have used defcustom as follows

(defcustom px-frame-initpos (cons 8 8)
  "Position of initial frame."
  :type 'sexp
  :group 'px-frame)

Then inside some function, I want to reset the value of "px-frame-initpos".
Have been using 

(custom-declare-variable 'px-frame-initpos (cons lft top) "DESC")

Perhaps using "custom-declare-variable" is not the appropriate thing to do, as 
it seems 
that it is declaring the variable from scratch, needing also ":type" and 
":group".