I have changed my profile "About me", but when I click on "Network Profile" there is still the old text.
How can I change the "about me" in my network profile?
The network profile is updated in a different way from per-site profiles. It is synced with your oldest site profile once in 24 hours. When you access your network profile, you should see something like this under the "about me" box (it's not visible to other users):
The words "Sync with..." are a link. If you don't want to wait for scheduled sync, click that link. It will immediately copy the content of the site's profile to the network profile.