Your Yousician subscription works in all Yousician apps. Just use your Yousician login details to log in and all your progress and subscription details are transferred automatically.
You can switch between guitar, piano, ukulele, bass and singing curricula by selecting the instrument so that it turns green under Game → Instrument.
To access piano, download the Piano by Yousician app from iTunes or Google Play depending on your device.
To access ukulele, download the Ukulele by Yousician app from iTunes or Google Play depending on your device.