Upper octave needs more ‘energy’ than lower octave. That is, the air has to go through the flute faster. You can do this two ways. One is by blowing ‘harder’, that is, by pushing more air through the same sized opening in your lips. The second way is by making your embouchure smaller, ie, pushing the same amount of air through a smaller sized opening in your lips. Both ways make the air go faster.
You have to do either/both enough so that the upper octave is produced. In fact, the upper octave is already present in the lower octave, so another way of thinking of the process is that you have to increase the ‘energy’ of the air stream to the point that the lower octave drops out so that you hear the upper octave.
Making the opening in your lips smaller is more difficult for beginners than blowing more air through the flute, because beginners haven’t developed enough strength and control in the lips/embouchure. But they (you) should try. Even by trying and failing to get the upper octave using embouchure control instead of just blowing harder will still develop the muscles.
Changing the angle that you blow into the flute sometimes helps to produce the upper octave, by helping to make the opening smaller. But this method is never going to help you get a good, easy upper octave. You need muscle control to do that and the only way to get stronger and more controlled muscles is to use them.
The main thing is to keep trying. Ask anyone who can produce ‘kan’ consistently and easily, how many hours she or he’s spent blowing into her flute and you’ll get an idea of why she or he can get the upper octave easier than you. But not to worry; you won’t have to spend so many hours before you too can produce ‘kan’ easily.
One way to develop good muscle strength and control is to play every note, including the lower ‘otsu’ octave as if you were trying to get upper octave. Try to be on the edge all the time, producing a focused sound that is either in kan, if you want to be there or nearly is, even when you want to be in otsu.