Use your cursor to highlight text—as you would highlight text in order to copy and paste it (note you'll have to "check out" the document to access the editing tools menu).

Click the “Link” button in the center of the text toolbar.

Enter a URL in the text box.

Click the “Add” button.

You can confirm the link by rolling back over it and clicking on the pop-up link, or check the document back in to see how the link will normally display. 

To edit or change an external web link, erase the link text (using the backspace/delete functions on your machine) and rewrite your text and create a new external web link.