在 CentOS 系統中，yum upgrade 與 yum update 都會將系統包更新到最新版本。

至於區別，我們還是通過 man yum 的幫助資訊來了解也許更好：

update If run without any packages, update will update every currently installed package.  If one or more packages or package globs are specified, Yum will only
              update  the listed packages.  While updating packages, yum will ensure that all dependencies are satisfied. (See Specifying package names for more infor‐
              mation) If the packages or globs specified match to packages which are not currently installed then update will not  install  them.  update  operates  on
              groups, files, provides and filelists just like the "install" command.

              If the main obsoletes configure option is true (default) or the --obsoletes flag is present yum will include package obsoletes in its calculations - this
              makes it better for distro-version changes, for example: upgrading from somelinux 8.0 to somelinux 9.

              Note that "update" works on installed packages first, and only if there are no matches does it look  for  available  packages.  The  difference  is  most
              noticeable  when you do "update foo-1-2" which will act exactly as "update foo" if foo-1-2 is installed. You can use the "update-to" if you'd prefer that
              nothing happen in the above case.

upgrade
              Is the same as the update command with the --obsoletes flag set. See update for more details.
 

根據幫助資訊可以看到，upgrade 與附帶 --obsoletes 的選項時的 update 是一樣的，

重要說明如下：

upgrade:

Is the same as the update command with the --obsoletes flag set. See update for more details.

update:

If the main obsoletes configure option is true (default) or the --obsoletes flag is present yum will include package obsoletes in its calculations

- this makes it better for distro-version changes, for example: upgrading from somelinux 8.0 to somelinux 9.

然而， update 即使不附帶 --obsoletes 選項時，預設的配置中將其設定為了 true （開啟），在 /etc/yum.conf 檔案中可以檢視到預設的配置資訊：

[main]
cachedir=/var/cache/yum/$basearch/$releasever
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
installonly_limit=5
bugtracker_url=http://bugs.centos.org/set_project.php?project_id=23&ref=http://bugs.centos.org/bug_report_page.php?category=yum
distroverpkg=centos-release
 

所以，只能說預設情況下沒有區別，而使用 update 則更為靈活。

參考：
in-centos-what-is-the-difference-between-yum-update-and-yum-upgrade
CONFIGURING YUM AND YUM REPOSITORIES

轉載至https://www.jianshu.com/p/4df7692bdc2b