Note: I struggled with making this work for a while, so I’m mostly posting this as a reminder to myself for the next time I need to do it.

If you’re running WordPress Multisite with a self-signed SSL certificate–such as on a local development site–you’ve likely seen an SSL error when trying to upgrade your network after a WordPress upgrade. In order to work around this, create a file in your wp-content/mu-plugins directory1–I called mine network_upgrade_fix.php— and put the following in it:

Save the file and run the network upgrade again and it should work.

  1. You may have to create this directory if it doesn’t exist already.