If it's the spelling for everyone in your database who has that surname, use the Surname Master List to make a global change. That's View > Master Lists > Surname...
Otherwise, you'll need to take more care to only change the right people. I'm not sure what the best way to go about it is - perhaps someone else here can chime in? It might be tagging, or Search and Replace...
If you don't find what you need here, you might like to subscribe to the Legacy User Group mailing list and ask there - it's very helpful, with many experienced people contributing when they can - including Legacy staff. Go to http://www.legacyfamilytree.com/LegacyLists.asp
to sign up.
Hope this helps. :-)