upgrade to fedora 31

• update existing fedora 30

dnf upgrade --refresh

• Install system upgrade plugin

dnf install dnf-plugin-system-upgrade

• upgrade to fedora 31

dnf system-upgrade download --releasever=31

• After waiting for the download to complete, an error may be reported, which the official did not mention.

警告：/var/lib/dnf/system-upgrade/fedora-39217cb7243e0d8e/packages/dnf-4.2.9-5.fc31.noarch.rpm: 头V3 RSA/SHA256 Signature, 密钥 ID 3c3359c4: NOKEY
Fedora 31 - x86_64 - aliyun                                                                                                                                                                                                                                       0.0  B/s |   0  B     00:00    
Curl error (37): Couldn't read a file:// file for file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-x86_64 [Couldn't open file /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-x86_64]
下载的软件包保存在缓存中，直到下次成功执行事务。
您可以通过执行 'dnf clean packages' 删除软件包缓存。
[root@dev5 ~]# rpmkeys --import /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-x86_64
错误：/etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-x86_64：导入读取时失败(2)。

• After running the command below, you dnf system-upgrade download --releasever=31can then.

rpmkeys --import /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-31-x86_64

• Reboot the system.

dnf system-upgrade reboot

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