You can do this using a simple find and replace.

Click on "Edit", then select "Replace"

When the Find and Replace box pops up, there's a button on the bottom that says "More" and has two little black triangles. If you click on this button, a bunch of check boxes will appear. Under those check boxes there is another section that says "Replace" and one of the buttons is "Format". It has one of those black triangles.

Click the "Format" button and the very last option in the menu is "Highlight".

Click on "Highlight". Now underneath the box that says "Find what", it will say "Format: Highlight"

Click inside the "Replace with" box, and click the "Format" button then select "Highlight", then do it again. When you select "Highlight" twice, you will see that underneath the "Replace with" box, it says "Format: Not Highlight".

Then all you need to do is click "Replace All"

Of course, it might be best to test it first by clicking "Find Next" and then "Replace" to make sure you get the desired results before clicking "Replace All"

Hope this helps!