It is a multi-step process.
First DJI must release the SDK that supports a given drone. This has just occurred for the MM.
Then the developer of a given app (in this case Litchi) must use the updated SDK so that they can then support that given drone (in this case the MM). How long this process takes is completely up to the developer. It can take a few days to a few months or more, depending on the complexity of the SDK and the resources of the developer.
The only thing that can be done is to wait until Litchi announces they can now support the MM.