We've simplified the interface and processes around managing integrations with CRMs - there are some further updates to come here, as we're currently integrating with a set of CRMs that are expected to come online in July-August.
Key points:
Navigate to Settings, Access & Permissions, Integrations - you'll see a list of CRMs who have integrations with Rello that you can access
You can easily enable/disable any integration via the link provided - remembering that you'll still need to provide your "M-ID" to the CRM so they can enable things on their side
You can also create your own API access tokens for integrations with any system not in the partner list