When you have written something in italics, you see that the word starts and ends with an i surrounded by square brackets. To continue with normal text, you just press the arrow key until you are outside (to the right of) the last i in square brackets.
Alternatively, you write the text without any spesific editing, then you just mark the part you want in italics, then press the button for italics to make the wanted change.
I have done it now on my iPad to see if it works. And it did.