centos7.9 降级 7.5
下载 Centos 7.5 的rpm包
wget --no-check-certificate https://www.repo.cloudlinux.com/cloudlinux/migrate/release-files/centos/7/x86_64/centos-release-7-5.1804.el7.centos.x86_64.rpm
安装
rpm -Uvh --oldpackage centos-release-7-5.1804.el7.centos.x86_64.rpm
重启
reboot