Selecting text, the normal way. Tap and hold on a word, and you will get a popup menu with Select, and Select all as options. {lift your finger when you see the magnifying glass} Choose select, then drag the little dots (drag handles) to include the text you want to replace. Enter dictation mode while the text is highlighted and it (that text) will be replaced by what you say.
You can also double tap on a word to highlight just that word, then drag the handles to expand the text. This works best with larger text fonts.