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$('tw-sidebar tw-icon[title="Undo"]').text("Back").attr("title", "Back");
<script>$('tw-sidebar tw-icon[title="Undo"]').text("Back").attr("title", "Back");</script>
Now we come to the harder part, making the new undo/redo buttons look good. If you have tried the above suggestions you will notice that the new buttons may overlap the passage text area because the text being displayed is longer than a single character/symbol. If your proposed replacements are also a single character/symbol then there should be no issue otherwise you will need to use CSS to style the new buttons correctly.