Just in case you need some more specifics:

You need the advanced find/replace (CTRL-H). Put your cursor in the FIND box. Before you type in the text,

Click on the button that says MORE
Click on the button that says FORMAT
Click on FONT
Select ITALIC

Now type the text you need to find. Notice that under the find box, it will mention italic text

Now put your cursor into the REPLACE box, and repeat the FORMAT, FONT, and choose regular or Not italic

By the way you could use the CTRL-I key, instead of the more button. CTRL-I once chooses italic, CTRL-I twice turns it off