Upgrading from 10.3 to 12

I would like to do an in-place install/upgrade to FreeBSD 12. How might that be (safely) done, since freebsd-update doesn't work and FreeBSD 12 isn't on the subversion sites?
12 is on the Subversion sites, but it is called "head" until it is released. Likewise with freebsd-update(8), it won't be available that way until release. If you're not comfortable with building from source, running head is probably going to be uncomfortable.
I'm comfortable with buildworld and buildkernel, but I'm still stuck on getting the source out.

Sorry if I'm being dense, but this:

svn checkout [URL]https://svn0.us-west.freebsd.org/base/head[/URL] /usr/src

simply checks out 10.3-RELEASE sources.

What URL should I be using then since:


is invalid?