Wei Wang, i'm sorry to say this but Kentico Xperience 13 is a completely different animal than previous versions. You can't simply 'upgrade' from 11 to 12 to 13. You have to rebuild your entire website in .Net Core.
It is possible to carry some data (page structure) but existing widgets will not translate over, and it's a tricky processes to do, you'll probably need help doing it if you do need to upgrade.
My recommendation is to upgrade your Kentico 11 to 12 at maximum, it was the highest supported version that allowed portal engine, and then start looking to rebuilding the site in KX13 from scratch.