开篇直拳：所用环境为centos7版本，是腾讯服务器的一个，来我们开始吧
首先，备份
[root@VM-0-17-centos ~]# cd /etc/yum.repos.d/#切换到对应目录
[root@VM-0-17-centos yum.repos.d]# ls#养成好习惯，更改前先查看一下文件夹内容
[root@VM-0-17-centos yum.repos.d]# mkdir repo_bak
[root@VM-0-17-centos yum.repos.d]# mv *.repo repo_bak/
下载新的centos-base.repo到/etc/yum.repos.d/中，这一步很多博客都有，但我根据一些博客进行更新却出错，如下：
[root@VM-0-17-centos yum.repos.d]# wget http://mirrors.aliyun.com/repo/centos-7.repo
--2021-12-06 10:11:02-- http://mirrors.aliyun.com/repo/centos-7.repo
Resolving mirrors.aliyun.com (mirrors.aliyun.com)... 180.97.148.115, 58.215.146.116, 218.94.206.226, ...
Connecting to mirrors.aliyun.com (mirrors.aliyun.com)|180.97.148.115|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2021-12-06 10:11:02 ERROR 404: Not Found.
如上所示，它说http请求被拒绝，当时我还以为是我的服务器配置有什么问题，还给/etc/resolv.conf添加了233.5.5.5的DNS，重新跑一次，却发现还是类似的错误
[root@VM-0-17-centos yum.repos.d]# wget http://mirrors.aliyun.com/repo/centos-7.repo
--2021-12-06 10:11:41-- http://mirrors.aliyun.com/repo/centos-7.repo
Resolving mirrors.aliyun.com (mirrors.aliyun.com)... 58.215.123.238, 122.225.217.244, 122.225.209.205, ...
Connecting to mirrors.aliyun.com (mirrors.aliyun.com)|58.215.123.238|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2021-12-06 10:11:42 ERROR 404: Not Found.
（为了提供参考，我就放出来更改后的测试了）
后来想起http404的错误原因，怀疑是我的url有问题，所以进阿里云开源镜像站去复制链接，发现果然是链接有问题
[root@VM-0-17-centos yum.repos.d]# wget https://mirrors.aliyun.com/repo/Centos-7.repo
--2021-12-06 10:16:43-- https://mirrors.aliyun.com/repo/Centos-7.repo
Resolving mirrors.aliyun.com (mirrors.aliyun.com)... 124.238.244.242, 124.238.244.248, 124.238.244.236, ...
Connecting to mirrors.aliyun.com (mirrors.aliyun.com)|124.238.244.242|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 2523 (2.5K) [application/octet-stream]
Saving to: ‘Centos-7.repo’
100%[======================================>] 2,523 --.-K/s in 0s
2021-12-06 10:16:43 (391 MB/s) - ‘Centos-7.repo’ saved [2523/2523]
可以看到的是，它采用的是https连接方式，所以http请求理所当然地被拒了，我们经常有很多资料或者书籍可以查看，但它们的实时性却不一定得到保证，所以大家还是要抱着审慎的态度为好。
接下来是清除一下缓存并生成新的缓存，由于我查阅博客想要有个结果对照，很多都只粘贴个命令使得一些需要参考的地方没有参照效果，所以我这里反其道而行之。
[root@VM-0-17-centos yum.repos.d]# yum clean all
Loaded plugins: fastestmirror, langpacks
Cleaning repos: base extras updates
Cleaning up list of fastest mirrors
Other repos take up 291 M of disk space (use --verbose for details)
[root@VM-0-17-centos yum.repos.d]# yum makecache
Loaded plugins: fastestmirror, langpacks
Determining fastest mirrors
* base: mirrors.aliyun.com
* extras: mirrors.aliyun.com
* updates: mirrors.aliyun.com
base | 3.6 kB 00:00
extras | 2.9 kB 00:00
updates | 2.9 kB 00:00
(1/10): base/7/x86_64/group_gz | 153 kB 00:00
(2/10): base/7/x86_64/primary_db | 6.1 MB 00:00
(3/10): base/7/x86_64/filelists_db | 7.2 MB 00:00
(4/10): base/7/x86_64/other_db | 2.6 MB 00:00
(5/10): extras/7/x86_64/primary_db | 243 kB 00:00
(6/10): extras/7/x86_64/other_db | 145 kB 00:00
(7/10): extras/7/x86_64/filelists_db | 259 kB 00:01
(8/10): updates/7/x86_64/filelists_db | 6.9 MB 00:01
(9/10): updates/7/x86_64/primary_db | 13 MB 00:01
(10/10): updates/7/x86_64/other_db | 881 kB 00:00
Metadata Cache Created
安装EPEL源
yum install -y epel-release
这里显示没有什么可更改的，而且。。。。。。（被我后面的操作刷新过去了，没记录下来）
重新清除缓存并生成缓存
yum clean all
yum makecache
查看所有yum源
[root@VM-0-17-centos yum.repos.d]# yum repolist all
Loaded plugins: fastestmirror, langpacks
Loading mirror speeds from cached hostfile
* base: mirrors.aliyun.com
* extras: mirrors.aliyun.com
* updates: mirrors.aliyun.com
repo id repo name status
base/7/x86_64 CentOS-7 - Base - mirrors.aliyun.com enabled: 10,072
centosplus/7/x86_64 CentOS-7 - Plus - mirrors.aliyun.com disabled
contrib/7/x86_64 CentOS-7 - Contrib - mirrors.aliyun.com disabled
extras/7/x86_64 CentOS-7 - Extras - mirrors.aliyun.com enabled: 500
updates/7/x86_64 CentOS-7 - Updates - mirrors.aliyun.com enabled: 3,175
repolist: 13,747
可以看到，我们的yum源已经更替成了aliyun的了（mdzz，忘记一开始就查看了，不过你要是跟着做下来你也和我一样了，哈哈哈）
更新yum
yum -y update
这样一来基本就可以了。