With the cursor in the Find What box, press Ctrl+I once, so the label
below the box says "Font: Italic". Move the cursor to the Replace With
box and press Ctrl+I twice, so its label says "Font: Not Italic".
Leave the boxes themselves empty, and click the Replace All button.
If it's important to limit the replacements to italicized words that
are 10 pt and not any other sizes, then alter this slightly. Click the
More button to show the bottom half of the dialog. Click in the Find
What box, then click the Format button at the bottom of the dialog,
choose Font from the menu, and select 10 pt in the Font dialog. That
will also be shown in the label below the Find What box along with the
Italic notation. You don't have to specify the size for the
replacement if it's the same as the size being searched for.