Joseph448, you need to use SFTP or some other direct filesystem access to upgrade the plugin manually.
First, unzip the github files on your own machine, which should create a folder named wp-multi-network-master.
Then use SFTP (or whatever) to go to your server and find the wp-content/plugins/wp-multi-network directory, which is where the current plugin files are.
To be safe, copy those files down to your own computer (you probably won’t need them, but the first rule of the sysadmin is “Don’t do what you can’t undo.”)
Finally, copy the contents of the wp-multi-network-master folder on your computer into the wp-multi-network folder on the server, overwriting the files that already exist there.
Then see if your dashboard works. You may need to change file ownership or permissions on the new files so dashboard-based updates of this plugin will work in the future. See your hosting provider’s help resources for that.
If you aren’t confident you understand any part of these directions, I recommend talking to your host’s technical support people instead. Send them the github address so they know exactly what they need to copy.
Hope that helps.