As the title mentions, I want to change my about box on the main portal site (stackexchange.com), it did copy it from my Stack Overflow account but, well, even after I changed that it stayed the same. I'm confused...
The info at the top of your SE user page is pulled from your oldest profile, which, in your case (and most cases), is your StackOverflow profile. And, as Rebecca mentioned, it's cached for 24 hours.
So, as it currently works, you can change your SE about-me by changing your SO about-me, and then clicking "sync with oldest profile" on your SE user page.
The data is cached for 24 hours. Give it a bit of time and then check again.
Edit, I just looked and the info has since been updated.