In MapViewer 8 you can use the Home | Data | Reload command to update the map. In MapViewer 7, you can use the File | Reload Data command. Older versions of MapViewer did not have the Reload Data command. In versions prior to MapViewer 7, go to Map | Map Properties to open the properties dialog for the thematic map. In the properties dialog, you will see the name of the data file being used for the map. At the end of that line is an open folder icon that lets you change the data file. Click that icon and choose the same file name and worksheet as before. Then click OK to close the properties dialog.
Closing the map and reopening it also causes the data to be reread and the map updated, unless the map is a pin map. In that case, see the article Why doesn't my MapViewer pin map update when I reopen it after changing the data?
Updated October 6, 2016