What happens after I get a negotiated offer?

[penguuuu]

Changed from double degree of commerce/science to a single degree of commerce at monash so that I could do a double major. Do I still enrol and pick subjects for monash or do I wait till later and do I go to the enrolment day next week?

And how do I decline the first round offer of the double degree (I actually already accepted & made account ect)