Its better to give ur phone to Nokia Care. If it is within warranty, they will update for free, otherwise they charge a nominal fee. If something happens to ur phone while u are updating ur phone at home, you have to send it to service centre and Nokia is not to be held responsible while if something happens while it is being updated in Nokia Care, they will fix the phone for you for free.
In the Nokia Software Updater, it mentions in the minimum requirements that a broadband connection is required. So do keep that in mind.