Hi there, Adelle uses the Genericons icon set. I don't see an Instagram icon, but there is a camera icon that might work.
If you'd like to try it, add this code under Appearance > Custom Design. Click the CSS tab and add this completely below the existing green comments:
.pinterest-link a:before {
content: '\f102';
}Click Save Stylesheet when you're done.
Keep in mind that users will still see the word "Pinterest" when hovering over the link, because that's not something that can be changed with CSS.