Find And Replace Text plugin allows you to install a find & replace button to your WordPress Editor.
It supports batch finding and replacing string when you edit your posts either in TinyMCE visual or text mode.
Please note the plugin in TinyMCE visual mode also find and replace string in raw file. Which means that you should make sure there are not HTML tag to find and replace.
For example, if you find and replace « href » to « something else » in Visual mode. The plugin also change every « href » to « something else » if the HTML codes have links with « href » even you can not see the « href » string on the visual mode screen.
So please make sure that you are not replacing HTML tags in Visual mode. Or you can start from Text mode.
Remember saving your post before your find and replace anything. So even you make something mess you can always get revisions back.