Upgrade existing UI of app

I'm looking to upgrade the existing UI of an existing app, and I have a very clear vision in mind. The existing functionality of the app needs to be modified and the end product needs to support Android devices. I do have an existing design/styling guide that will need to be used as part of the project. As such, I'm looking for a qualified individual or team who can develop this upgrade with an eye for the user experience and aesthetic. The successful completion of this project will involve modifying the existing design and functionality to create an improved version with an intuitive layout and attractive visuals. The desired outcome is an app that will be appealing to users, efficient and effective.