The easiest way is to add the sharepoint calendar to your outlook then when you update the calendar it will update in sharepoint. Here are then instructions form Micrisoft.
To add a Windows SharePoint Services calendars to Outlook, follow these steps: 1. If the Windows SharePoint Services folder has not been added to Outlook, add it to the navigation pane. To do so: a. In your Web browser, locate the Windows SharePoint Services calendar that you want to access in Outlook.
b. Click Link to Outlook.
c. When prompted, click Yes in Outlook 2003 or click OK in Outlook 2007 to add the Windows SharePoint Services calendar to Outlook.
If you do not see the calendar that you shared in step 1 in the Calendar view, go to step 2.
2. In the Calendar area of the navigation pane in Outlook 2003, click to select the check box of the calendar name for the Windows SharePoint Services calendar that you shared in step 1 under Other Calendars.