Simply select some text by left clicking with your mouse and dragging your cursor over the text. Release the mouse when you have selected the text you wish to highlight. Then an inline toolbar will pop up. Select the highlight button. The text should now be highlighted.
Tap and hold the text where you wish to start your highlight until a magnifying glass symbol appears and selection buttons appear. Drag the blue dots to redefine the selection area. Select the highlight option and the text will be highlighted.