yum 查询多版本，降级软件包

# yum search --showduplicates salt

...

salt-2015.5.10-2.el6.noarch : A parallel remote execution system

salt-2016.11.3-1.el6.noarch : A parallel remote execution system

# yum downgrade salt

降级指定版本后面写查询到的软件包全名。