can’t undo highlights
Highlights made in microsoft word odt and doc files can be undone in libreoffice writer 4.2. Highlights in docx files can only be undone in 4.3.
For version 4.2: If it is the only formatting of the word, you can use
Clear Direct Formatting from menu Format or from the context menu. You can use it in the other cases too, but it will remove all formatting, not only the highlighting.