While Microsoft Outlook itself includes no search and replace functions, you can use a grouped List view to help update a group of items. This example updates Contact items to a different Company Name, but you can use this technique to update any field other than Categories. (You can use this method to add categories to a contact.)

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Using a Group By view

1. In Contacts, create a List view grouped by Company Name if you don't already have one.
2. Choose View | Expand/Collapse Groups | Collapse All.
3. Expand the group that contains the company whose name you want to change.
4. Open the first contact in the Group.
5. Change the Company Name to the new name.
6. Save the contact. It will end up in a new group.
7. Collapse the group that you expanded, containing the remaining contacts you want to change.
8. Drag the gray bar containing the original company name to the gray bar containing the new company name. If you hover briefly over the new company's group, you'll see a tip pop up, "Change Company Name to" whatever the group's name is.
   
9. Release the left mouse button.

See Tools below for add-ins which can do a search and replace.

Using in-cell editing

Although not a true global search and replace, you can use in-cell editing to edit individual contacts while in a list view. In-cell editing is typically enabled by default and you just need to click in the Contact's field (cell) to edit it. If in-cell editing is not enabled, enable in-cell editing in View Settings, Other Settings (View ribbon) or using the Customize Current view on the View menu in older versions. You can also right click on the row of field names and choose Custom (or View Settings in Outlook 2010 and up) then Other Settings.

Notes

This procedure can be used for other fields as well. Right mouse drag will give you more options.

If you need more flexible search and replace, try exporting from Outlook to a comma-delimited file. Open the file in Excel, and perform the search and replace there. Then re-import it into Outlook.

If you use this method to try to change the Category field, the new value is added to the categories for the items. It does not replace the category. Instead of using the grouped technique, try selecting the items you want to change, then right-clicking them and choosing Categories.