How to select paragraphs with similar style in Pages 5?
As far as I know, there is no “Select All Uses of …” command in Pages 5.

You don’t need that command to edit the format of all a style’s uses, though. When you change a paragraph's style, like changing some “Body”-styled paragraph’s font, an “Update” button will appear beside the style name in the format sidebar, and the style name will append a * to indicate that it was modified. Click the button and all paragraphs using that style will update.