Mail Merge with Labels, Update Labels button problem

I am using MS Word 2016 in Windows 10. Trying to use Mail Merge to create mailing labels. When I edit the format of the first label in the upper left corner of the sheet of labels and click on Update Labels, the entire sheet of labels is supposed to be updated with the new format. That's not happening. Instead, the label I edited resets to its prior format and my edits are lost. I did this myself, and I did it using the Wizard. Using an Excel spreadsheet as my data source for the names and addresses. Any help in figuring out why the Update Labels button is not working would be appreciated.