I would think it was the other way around:Yes that sentence is right.
Passive voice I think it would be: I have improved my English.
Yes it is correct.My English has improved. (Present perfect in Active Voice)
Is this sentence right?
If right, can I change this sentence into passive voice?
Yes it is correct.
To clarify since the proper passive structure is buried after a number of various active structures:
To make it passive: My English has been improved.