Carol, to alter one place name, you just access it in the EDIT INDIVIDUAL screen or the EDIT MARRIAGE screen. Click the text shown in the box and it should turn green. Click DELETE and it disappears. Type in your new place name and click YES if it is correct then click SAVE.
To replace an entry that is incorrect in many entries, you can do a "Global Search and Replace", with extreme caution. Click TOOLS|GLOBAL SEARCH & REPLACE. In the text box that opens, type in the incoorect place name in the top box and the correct place name in the bottom box. Just be aware that it will change ALL instances where the incorrect name appears in PLACE names. So attempt this with caution. You will see a text listing pop up with all the changes you have made.