Because you have not said why someone said that the second is not properly constructed, it is hard to answer your question.
method to improve
method for improving
are both grammatically acceptable.
EDIT: Based on a comment from the OP, I suspect that part of what was being asked was to discuss
What method of xxx is most suitable for improving yyy
What method of xxx is more suitable for improving yyy
The first sentence implies that there is more than one method, but does not imply that there are only two methods. The second sentence implies that there are exactly two methods. Both are grammatical, but they are not both applicable in the same context.