For a complete list of all the fields you may want to update and what values are appropriate, view the field list or look at the object directly in salesforce via setup / app setup / customize / {object name} / fields  for standard objects, or setup / app setup / create /  objects /  and select the object in question.

The full information on all an object's fields can be found there.

It is very important that you know the fields you have to work with, their picklist values, and how they are related.

For example if you are looking to update the Populations Served for the organization records, then make sure the values you are entering in the table to update those records are the correct picklist values in the system:

Example:  There is a picklist value "Visitors & Travelers" which is a valid picklist value.

Do not update this field in any of the records to "Vistitors and travelers"  -- it is not the same thing at all and will mess up search and reporting.

If you can run a report you can update records.  

The most important field when creating or running a report is to make sure that the Salesforce.Com ID is present in the export.  

The Salesforce.com ID is the value that is use to prevent duplication of the records in the system, especially for custom objects like Volunteer Opportunities.

See this post on how to determine which is the Salesforce.com ID

Using the Salesforce ID will ensure that you don't accidently create duplicates of existing data.

Once you have the correct Salesforce.com ID, then you can add additional fields to the report so you can ensure that you are dealing with the right record.  

(i.e.  the ID number won't mean anything to you, so you may want fields like Volunteer Opportunity Name, or other things you can interpret -- to ensure you are working with the right records.

Finally -- add the fields you wish to update to the report.  Even if they don't have values now,  you'll have all the columns you need to work with in the exported report.

This report is the BEFORE value of your data.

You'll export the .csv file, and then edit the.csv file so that the new field values you wish to have are present!   This is the way you want your data to look AFTER the update.

You'll import this file back into Salesforce to get the changed values to be put into the system.

When you import the file in you'll make sure the RIGHT records are updated, and that no duplicates are being created.

Caution:

Before you do an import of new records make sure that you have done your homework to ensure that the records you are importing don't already exist in your system.   Especially when importing contacts and accounts.  

For salesforce to find any duplicates based on Contact name, the contact has to be associated with the same organization record.

For example John Smith related to the organization Individual is not seen as the same John Smith associated with IBM, even if they have the same home address, phone number or email address.   So to save yourself time merging records down the road, do your homework first to make sure you really are importing new records and not a duplicate.

Running a report on existing data will allow you to update those records in the system.

If you wish to also import NEW records, you'll need to do this as a separate step.

To be able to update existing records and add new records at the same time, you'll need the 'upsert' capability found in the external application "Apex Data Loader