How to migrate a domain owned by a multisig

I have a domain that is owned by a multisig.

What contract interactions do I need to make to migrate it?

You need to call reclaim() on the .eth registrar. The first parameter should be the keccak256 hash of the name without the .eth extension (Eg, keccak256('foo')). This is also known as the labelhash. The second parameter should be the address you want to control the name in the ENS registry. The transaction must be sent from the multisig that owns (is the registrant of) the name.