If you mean "How do you improve your Korean?", you might say 어떻게 하면 한국어를 잘할 수 있습니까? It is literally "If you do what does your Korean get better?", but it's really the same as "How do you improve your Korean?". To make it closer to your sentence, you could also say 한국어를 어떻게 잘할 수 있습니까?, but the other sentence sounds more natural. Note that 한국 = Korea, 한국어 (or 한국말 for spoken language) = Korean. And 개선하다 is a big word used about a system, official relationships, and other serious things. It doesn't sound natural for improving language skills. 잘하다 (do well, be good at) is the usual term we use for that.