Right-click any toolbar. Select customize. On the Commands tab select Macros
in the Categories list. Select your macro from the Commands list and drag it
to the toolbar yuou want.

Separately, that's a pretty lousy way to update the fields. You could just
use this line in place of what you currently have --

ActiveDocument.Fields.Update

But are you aware that your code (and this one-line equivalent) updates
fields only in the body of the document, not in headers, footers, textboxes,
etc? If you want to update all fields, wherever they are, use --

Dim pRange as Word.Range

For each pRange in ActiveDocument.StoryRanges
   Do
       pRange.Fields.Updatge
       Set pRange = pRange.Next
   Loop until pRange is nothing
Next