Get your minecraft_server.1.12.2.jar
from the root of your server, download the last bundled jetty 4.1 version https://repo1.maven.org/maven2/io/netty/netty-all/4.1.68.Final/netty-all-4.1.68.Final.jar
Open your server jar with 7zip and navigate to minecraft_server.1.12.2.jar\io\netty\
open the netty jar file you downloaded and go to netty-all-4.1.68.Final.jar\io\netty\
back in the minecraft server jar delete these 6 folders
now simple copy the same 6 folders from the netty package into the minecraft jar
throw the jar back into the root of your server and you are done!
Could you explain please what this will change for the current downloaded 1.12.2 server version and will this patch added to 1.12.2 and change it maybe to 1.12.3?
Would be nice to know because what whill happen if smth. goes wrong? Safte first ;)
Edit: What about the client side version? Anything to edit here to not run into any conflicts?