More formal:
* You have acquired knowledge.
* You have gained understanding.
* You have become knowledgeable.
* You have developed expertise.
* You have mastered the subject.
* You have attained proficiency.
More informal:
* You've picked it up.
* You've got the hang of it.
* You've figured it out.
* You've come to understand.
* You've gotten the gist of it.
* You've learned the ropes.
* You've grown wiser.
More specific:
* You have learned how to [do something].
* You have learned about [topic].
* You have learned from [experience].
The best way to say it depends on the context and the tone you want to convey.