When developing new software, third-party libraries are commonly used to reduce implementation efforts. However, even these libraries undergo evolution activities to offer new functionalities and fix bugs or security issues. The research community has mainly investigated third-party libraries in the context of desktop applications, while only little is known regarding the mobile context. In this paper, we bridge this gap by investigating when, how, and why mobile developers update third-party libraries. By mining 2752 mobile apps, we study (i) whether mobile developers update third-party libraries, (ii) how much such apps lag behind the latest version of their dependencies, (iii) which are the categories of libraries that are more prone to be updated, and (iv) what are the common patterns followed by developers when updating a library. Then, we perform a survey with 73 mobile developers that aims at shedding lights on the reasons why they update (or not) third-party libraries. We find that mobile developers rarely update libraries, and when they do, they mainly tend to update libraries related to the Graphical User Interface. Avoiding bug propagation and making the app compatible with new Android releases are the top reasons why developers update their libraries.