Minecraft 1.12.1 wont update
ERR_CONNECTION_RESET suggests that there are connection problems with the server. If the update just came out, it could be that the servers are overloaded and unable to process your update request at this time. Try to update at a different time.
If the problem persists, make sure that your antivirus software is not actively blocking the download, and that your firewall is allowing the connection to be made. This could also be the modem/firewall on the router itself.