RabbitMQ Upgrade Fails on Ubuntu

When updating RabbitMQ, you might encounter this issue if you have old version of RabbitMQ

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://www.rabbitmq.com testing InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6B73A36E6026DFCA

W: Failed to fetch http://www.rabbitmq.com/debian/dists/testing/InRelease

W: Some index files failed to download. They have been ignored, or old ones used instead.

RabbitMQ changed their PGP key as explained here. Download and install the new key and your update should work successfully.

$ wget -O- https://www.rabbitmq.com/rabbitmq-release-signing-key.asc | sudo apt-key add -