Try the following: 

• Open /lib/wysiwyg.php and save it as wysiwg_custom.php
• Edit /lib/wysiwyg_custom.php
• Search for "toolbar1"
• And the text " undo " where you want the undo button to appear.
• Optional: Refer to the button reference here: https://www.tiny.cloud/docs/advanced/editor-control-identifiers/#toolbarcontrols

Hope that helps!

Thank you.