Hi Maria,
Once your service upgrade has been scheduled, you’ll see a "Postpone" button appear on the service upgrade options pages. You will have about three weeks from your initial scheduling notification to postpone. (Check the email: the deadline for postponing is at the end). After you submit a request to postpone the service upgrade, you won't hear from us again for at least a month.
For more information about the Postpone, please see:
http://community.office365.com/en-us/wikis/upgrade/postpone.aspx
In addition, after your Office 365 subscription is upgraded, site collection administrators can upgrade individual site collections to make new SharePoint Online 2013 features and user interface improvements available. It means you can upgrade the site collections manually.
For more information about how to upgrade a site collection, please see:
http://office.microsoft.com/en-001/sharepoint-help/upgrade-a-site-collection-HA102865473.aspx
Thanks,
Keats Ge