Execute the following commands when upgrading the flutter

Flutter upgrade

Unfortunately, for some reason, it cannot be accessed. The following error is reported

Standard error: fatal: unable to access
'https://github.com/flutter/flutter.git/': LibreSSL SSL_connect:
SSL_ERROR_SYSCALL in connection to github.com:443

You can set the routing agent. The command is as follows:

git config --global http.proxy http://127.0.0.1:1080
git config --global https.proxy http://127.0.0.1:1080

Note that the port can be modified according to its own configuration. Set or not set HTTPS according to your needs
if you want to cancel the proxy setting, you can use the following command:

git config --global --unset http.proxy 
git config --global --unset https.proxy

After setting, test OK

Git config — the global command will add the following configuration in the ~ /. Gitconfig file

[http]
	proxy = http://127.0.0.1:1080
[https]
	proxy = https://127.0.0.1:1080

Note:
there is another way to use this command. It has not been tested yet. Save it first

git config --global --add remote.origin.proxy