How to convert an Interface.IUnknown handle to a known registrited COM object

I'm currently working on COM objects in Matlab, I can well create a COM object with

retObj = actxserver('xxx')

When I called a function of 'xxx':

retInt = GetAnInterface(retObj)

It gives a result of type

Interface.IUnknown

Because Matlab does not know the exact type of `retInt`, I cannot use any method of retInt.

According to the tutorial of 'xxx', in other languages, it is necessary to declare a variable of type 'yyy' and then the com interface will be casted automatically.

My question is how to cast the retInt (Interface.IUnknown) to another COM object type? Thanks in advance.