learns (how) to relax ?

“The instructor of a class for people who are afraid of flying learns to relax.” Does this sentence mean that the instructor learns the people (who are afraid…) to relax ? Or does this sentence mean that the instructor himself learns to relax, and should one add the word “how” if one wants to state that the instructor learns the people “how” to relax ?

No. ’Learn’ does not mean ‘teach’. The instructor himself learns to relax, and the word “how” can be added optionally, though it is unnecessary.

It looks like a badly constructed sentence. The meaning of the sentence as it stands is that the instructor is learning to relax.
I suspect the verb ‘to learn’ has been used incorrectly in place of ‘to teach’.
This is likely to be the correct sentence:
“The instructor of a class for people who are afraid of flying teaches them to relax.”
This way the meaning is clear - the instructor is teaching the people to relax.

‘how’ can be used, but is not necessary. There is a slight difference in meaning if you use it. It would indicate more strongly that he is teaching them the techniques of relaxing.

Note the difference between ‘teaching’ and ‘learning’:
You teach when you pass on the knowledge to others.
You learn when you receive the knowledge.