FLStudioMobile Android:
Please check the Google Play Store support on this subject: Download previously purchased & downloaded apps For FLStudioMobile iOS:
Please check the Apple iTunes support web site on this subject: Downloading past purchases ...
FLStudioMobile app from the Windows app store:
You run Windows S at this moment. A free change to a regular Windows version is needed first, before you can install FLStudio from the links above. More ...