diff --git a/manual/edit.rst b/manual/edit.rst
index 0a67f7586f..3eddc930af 100644
--- a/manual/edit.rst
+++ b/manual/edit.rst
@@ -482,6 +482,14 @@ attributes. After removing the style information, a summary of all the changes
 made is displayed so you can see exactly what was changed. The tool can be
 accessed via :guilabel:`Tools->Filter style information`.
 
+
+Upgrading the book's internals
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This tool can be used to upgrade the book's internals, if possible. For
+instance it will upgrade EPUB 2 books to EPUB 3 books. The tool can be
+accessed via :guilabel:`Upgrade book internals`.
+
 .. _checkpoints:
 
 Checkpoints
diff --git a/src/calibre/gui2/tweak_book/ui.py b/src/calibre/gui2/tweak_book/ui.py
index 22b989a5f3..2f09ec7be9 100644
--- a/src/calibre/gui2/tweak_book/ui.py
+++ b/src/calibre/gui2/tweak_book/ui.py
@@ -424,6 +424,9 @@ class Main(MainWindow):
         self.action_remove_unused_css = treg(
             'edit-clear.png', _('Remove &unused CSS rules'), partial(
                 self.boss.polish, 'remove_unused_css', _('Remove unused CSS rules')), 'remove-unused-css', (), _('Remove unused CSS rules'))
+        self.action_remove_unused_css = treg(
+            'arrow-up.png', _('&Upgrade book internals'), partial(
+                self.boss.polish, 'upgrade_book', _('Upgrade book internals')), 'upgrade-book', (), _('Upgrade book internals'))
 
         # Preview actions
         group = _('Preview')