See Microsoft Word Online Help for the following instruction:
Remove highlighting from part or all of a document
Select the text you want to remove highlighting from, or press CTRL+A to select all of the text in the document.
On the Formatting toolbar (toolbar: A bar with buttons and options that you use to carry out commands. To display a toolbar, press ALT and then SHIFT+F10.), click the arrow next to Highlight , and then click None.
You can get any help you'd like by clicking "Help" on the menu.