Write the rightsholder, which stands for either an individual or corporate author of the software, followed by the year, title of the software, version number, description of the form and from where you have got it.
Rightsholder. (year). Title of the app (Version number) [Mobile application software]. Retrieved from https://xxx
Skyscape. (2013). Skyscape Medical Resources (Version 1.17.42) [Mobile application software]. Retrieved from https://itunes.apple.com
Note: Android apps should have the similar address: https://play.google.com/store/apps
Read more about this on the APA Style website: Mobile App Reference Work