How can I edit the About Me section of my profile?

To edit your About Me, head to your Profile updater, and, under Marketing and Promotion, click on Bio. 
Make sure to click on the Save button below the Bio editor before leaving the page.

If you want to add the format to your bio, click on A WYSIWYG editor is also available. This editor will allow you to bold, underline, and italicize your text and insert links, headers, images, etc.

You can use this link to head to your Bio editor https://www.proz.com/settings/freelancer/marketing_promotion/about-me

If you want to add colors, tables, or other rich format features that are not available in the WYSIWYG editor, you can edit the format manually using HTML code. To do this, click on "Code view" and edit the HTML source. After you have finished editing the HTML, go back to the rich text view by clicking the same button, and then save your changes.

You can learn how to add colors, as well as other simple HTML attributes, with this HTML tutorial.