Ensure you are in Edit Mode located in the top left corner. Click here to learn more about Modes.
Click on the text you would like to highlight and a blue text box will open around the text. If you need help adding/modifying text click here. You will now be able to hold down the left mouse button and select the portion of text you would like highlighted, it will turn blue.
In the Font section of the Edit Module you will see familiar text tools. Choose the S with strikethrough applied and a color display will popup. Choose your color and you will see the underline appear.
You can also use the Forms tab's Strikethrough. Select this to underline text.
Choose the color of your choice and you will now see the text markup.
To remove it, right click on this text mark-up and choose Remove.