Hello, Sultan. It would be better to simply say, "improve weakness" or "improve upon (subject's) weakness". Weakness can be improved, but you do not need the article 'the' before it. I have included some example sentences to help you. 1. John was learning English and he studied frequently to improve upon his weaknesses. 2. Study this book to improve your overall weakness. I hope this answer helps. If you are interested in taking professional English lessons to increase your fluency, I am currently accepting new students. I would be happy to work with you!

I would say it is more ambiguous than incorrect. Does 'improving a weakness' mean making it more weak, or less weak? It's not clear to me.

@Thank you, Ms. Helga

@Helga: drAwback, flAw. 'weakness' might be the best word, depending on the context. We don't know what that is, so we can't tell if it is or not.

I think, you should use another word, instead of weakness. A drowback, or a flow. These can be fixed. Improving a weakness doesn't sound english, it's a literal translation from another language.