How I can update the Ntop application..?
If you have installed NS 7.3, you need to update to NS 7.4 beta to get the new webtop release.
I think the problem is, that ntopng webpage shows update message, but it's not supported, because they want money now.
See this thread:
Yes, you can pay the support but you can also update just upgrading NS to 7.4
While you're on NS 7.3, just ignore the message.