First, I accept that I, along with everyone else in the world, cannot know everything.
Next, if a subject/topic/etc. has no measurable impact on my life, I feel no need to address the unknown unknowns. For example, I have no idea what I don't know about scuba diving, but since I have never scuba dived, and have no plans to in the immediate future, I feel no need to find out. If that ever changes, I will speak to an expert.
Which brings me to topics/subjects that do affect me. In order to deal with unknown unknowns, I will speak to an expert or do research, beginning with an introduction to a topic, and branching out to sub-topics from there. I still won't know everything, but many of the unknown unknowns will have been transformed into known unknowns as I become aware of the different sub-topics, etc, of the subject in question. I will also look for parallels that I am familiar with; for example, I know nothing about the politics of New Zealand, but I'm very familiar with Canadian and American politics, and passingly familiar with Asian and Australian politics, so I have a basis to begin research on New Zealand politics, and from there I would look into their history and demography. There would still be unknowns, but at this point I would have enough knowledge to know what I don't know.
Unknown unknowns are only a problem for a) people who assume they know everything they need to know and b) predicting the future, which is impossible.
hope it's help<3