App development

I need an experienced mobile app and web development expert to work upon a comprehensive application which is already live on google play store for me. It will be developed for both the iOS and Android , so it will need to be user-friendly for all platforms. Additionally, it will require a backend as there will be data storage associated with it which is already there you just have to make some changes. I already have sketches or wireframes of the app ready, so all they will have to do is turn my vision into reality. The finished product should be feature-rich and robust, with a great UI/UX design.