[plt-scheme] Changing the value of an variable

I want to change the value of an variable. Until now I've always done it by
'(define a 4)'
but in this doesn't work in the following code:
'(define a 3)
(define (test n) (define a n))'

Which command should I use instead? Sry if this question seem very 
silly, but I didn't find the answer any where.


Mfg Matthias Redies

	
		
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