# freebsd-update -r 10.2-RELEASE upgrade
Looking up update.FreeBSD.org mirrors… 5 mirrors found.
Fetching metadata signature for 10.1-RELEASE from update5.freebsd.org… done.
Fetching metadata index… done.
Fetching 1 metadata files… done.
Inspecting system… done.

The following components of FreeBSD seem to be installed:
kernel/generic src/src world/base world/games

The following components of FreeBSD do not seem to be installed:
world/doc

Does this look reasonable (y/n)? y

Fetching metadata signature for 10.2-RELEASE from update5.freebsd.org… done.
Fetching metadata index… done.
Fetching 1 metadata patches. done.
Applying metadata patches… done.
Fetching 1 metadata files… done.
Inspecting system… done.
Fetching files from 10.1-RELEASE for merging… done.
Preparing to download files…
done.
Fetching 14034 patches…..10….20….30….40….
….
….
….12560… done.
Applying patches…
done.
Fetching 31109 files…
failed.