You can change all instances of one place name, but not all instances of all place names, in one operation.
There are two ways.
In an input screen (like the Person Tab of the People Workspace), find any person who has a fact with that place name that you want to change.
Now, go into that place name and change it and hit tab (or enter).
You will see a popup dialog box.
Do you want to change this name.
You will see two choices at the top of that screen:
1) Do you want to change all 23 places with this name
2) Do you want to change this place only.
Choose Number 1 and hit ok.
Now, is it resolved?
Look at the right hand end of the place name entry box to see if the name is resolved. If it isn't click on the question mark.
You can then resolve it the way you want, or hit cancel and leave it the way it is. (if it's the correct place name coming from a record I would leave it as it is).
The second way is to go to the Places Workspace and find the place name.
Choose to "Change" it, or "resolve" it.