IMPORTANT:

First, back up your QB Company File if that option is available. Otherwise consider doing an Item Export (see below) and storing that version as a backup. Then perform another export and work off this 2nd export copy.

How to Export all Items from QBO into Excel

Go to your Products & Services list.

In the top-right of the data grid is an export icon (looks like a square with an arrow). Clicking that will generate and download an Excel XLS file.

How to Import an Excel File into QB and Batch Update Items

On the Products & Services screen the green "New" button is an expansion arrow, clicking that will display an Import option.
Choose your updated file and proceed with the update.

Notes / Suggestions

The QB import Items flow / UX can be difficult to use and confusing. Sometimes it will flip an Item to be taxable when it's not supposed to be, or cause some other strange discrepancies.

Personally I find using a 3rd party app that integrates with QB to have a better interface.

One example is an app titled "Import Excel | Google Sheets | CSV - Business Importer".

To find and install these apps click on Apps in the left-navigation then Find Apps. In the search apps box enter a query of "excel" and there will be a couple of results. 

One is titled "Import Excel | Google Sheets | CSV - Business Importer". Click on it's name to view details and if you want to proceed click on Get App Now

Once you have installed the app you launch it and use the interface provided.