Yes, that should be possible. You need to filter the tags first for the tag you wish to replace, Filtering is accomplished by clicking the funnel icon at the top of the column and selecting the tag - you can type it in or use the check boxes. At that point you should be able to replace all of the occurrences of that tag.
If your still having trouble let me know what tag your trying to replace and I can try coming up with some better help on this topic.
