The key to any data update is to build the correct report.  For this example we will be creating a new report.  From the Reports tab click on "Create New Report".  On the Create New Report, type in "Occurrences" to create a report based on the Occurrence object, then click "Continue".

In the report itself make the following changes:

1) Show Me: All Occurrences

2) Date Field: End Date & Time

3) End Date & Time: Whatever criteria you choose. Leave at All Time to see all occurrences, or if you are updating future occurrences, change the range to Custom, use today's date as the Start Date and leave End Date blank. Click Apply.

5)  Add Filter:  Enter whatever filtering criteria you'd like such as Volunteer Opportunity, Recurrence, Managing Organization, etc.   For this example we are filtering for the occurrences associated with the Volunteer Opportunity: Field Day Volunteers.

Volunteer Opportunity EQUALS Field Day Volunteers

The only column that is initially present is the Occurrence: Occurrence ID column.  You can leave this in the table for easier reference or remove it if you prefer.

6) Add these columns:

Occurrence: ID

Start Date & Time

End Date & Time

****Update all existing connections? In the event that the occurrences you are updating currently have active connections and you want the date information to be reflected on all of those existing connection, an additional data load must be made to those connections when using the Import Wizard.

7) Once you have all the columns and desired filters, click on "Run"  (You can save this report if you feel that you will be using it often to update occurrences and just modify the filters as needed the next time)

After you make the necessary updates to your data table, you need to ensure that the Date & Time fields are in the right format for salesforce to recognize the new values.

This note is from the Salesforce.com Help & Training post.  

When importing Date/Time values into Salesforce using the import wizard, you cannot format the cells using the 24 hour clock. The import wizard requires the AM and PM nomenclature in the field. Do this by formatting the cells so that the mask used is: m/d/yyyy h:mm AM/PM

To do this in Excel, follow the instructions below:

1. Highlight the column in question.

2. Right click and select the "format cells".

3. Select the "custom" option and place your cursor in the Type field on top of the list and add the value:
m/d/yyyy h:mm AM/PM

5. When done, click OK, and your column will be properly formatted.

We will use the Salesforce Import Custom Objects wizard.

Go to Setup then type Import Wizard in the quick find on the left.  

Then click Launch Wizard! to begin.

Click on Start Import and you will be greeted with a "Congratulations, your import has started!" window. Click OK and you will be automatically taken to your Bulk Data Load Jobs page where you can see the status of your job.