Henka means "variation" or "change". In other words, a different way of doing something. It doesn't refer specifically to the kihon happo ("basics" / "essentials") of Bujinkan Budo Taijutsu, although you are expected to learn many different henka for each technique. That's part of the training.
The theory goes like this: When a technique is first shown, it is taught in a sort of "textbook" manner so as to show all of the key features of it. Then the instructor typically shows a series of henka of the technique. In other words, you'll see it done differently. Then, after doing it a number of different ways, your subconscious mind will eventually figure out what the principles are, rather than having to learn rote technique. And from there, you will be able to apply the principles in any shape or form of technique on the fly as needed.
By the way, this doesn't happen overnight. It can take years, if not decades before students are just able to react subconsciously with the right technique. It just depends on the student, the quality of instruction, and how often the student is exposed to the techniques and concepts.
The advantage said of this method of training is that it doesn't rely on rote memorization of dozens of seemingly different, distinct techniques that all have the same principles. Instead, your brain absorbs the principles, and then the technique emerges from that. You may even make up a technique that nobody has thought of before, and yet it is just a henka of something else you've learned.
The funny thing is that Bujinkan teachers rarely just come out and tell you what the principles actually are. They just show you the technique and a grab-bag of different henka for it. They may speak of principles, but language itself isn't generally good enough. You have to feel it in order to understand it. At least according to what I know from my Bujinkan training days.
Watching videos is an incomplete way of learning anything in the Bujinkan. It can help. You can learn from it. And there are many youtube videos of Bujinkan instruction.
The downside to learning this way is that you won't be able to feel it being done to you, and so you won't really know if you've truly understood it.
Hatsumi Sensei once said this of learning anything in Bujinkan:
"The Uke is usually the only one who understands what’s happening. The people who stand around and watch might think they understand, but they don’t. All they see is what I show them. That part they can do themselves, and teach on to their own students. What Uke experiences cannot be replicated afterwards. Even if they truly feel it, they can’t explain it afterwards. I always ask people to explain the feeling back to me. This is how I know how good they are. Sometimes silence is the best answer. When I ask them to explain and they just shake their heads and laugh, that’s when I know they understand."
The only way to learn more henka for a given technique is to learn it from a qualified instructor and have it done on you.
Yes, you can take it apart mentally and try to figure out what the essence is of the technique. That can help. But I find that no matter how creative you are, you're just not going to come up with all of the many henka that Bujinkan people have shown over time. I know 15th dan black belts in Bujinkan that are still amazed every single time they train with Hatsumi-sensei or any of the Japanese shihan. They're constantly being shown new aspects of stuff they thought they understood completely already.
Bottom line: If you don't have anyone to train with near you, get to a seminar, a workshop, a Taikai, whatever. There are many, and they cover a wide range of techniques. You might have to travel a little, that's all. Or better yet, head to Japan if you can.
Hope that helps.