An experimental approach to improve the user interface of an application is A/B testing, where two groups of subjects are tested with different variations of a user interface. While A/B testing has quite a long history in web development, it is considerably more difficult to apply for native mobile applications. For websites, different versions of the user interface can be provided on the server, and the website traffic is simply split between them. For native mobile applications however, even the smallest changes like the relocation of a button requires to recompile and republish the application. This is a considerable drawback though, as it is more time-consuming and less flexible compared to A/B testing of web applications. In this paper, we present an approach for exchanging the user interface of native Android applications at runtime. It just requires to include a library in the application, which is able to load new user interface versions from a server.