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 ...
FLStudio for desktop. This allows you to transfer your MiniSynth presets to FLStudio and get the same sounds. Alternatively you can just use MiniSynth inside the FLStudioMobile Plugin that also runs inside FLStudio ...