The CloudAMQP Control panel has a new feature: version switching to an arbitrary RabbitMQ version. It looks like this:
You can upgrade to any higher version than the one you're currently running, but you can't downgrade.
We currently don't allow customers to upgrade to RabbitMQ 3.6 due to bugs in that version that leads to high CPU and memory usage. As soon as this issue has been fixed we'll allow it again.