You can switch between guitar, piano, ukulele, bass and singing curricula by tapping on the instrument so that it turns green under Game → Instrument.
When you want to access piano on a mobile device, you might need to download the Piano by Yousician app. You can find the app on iTunes and Google Play. You can use the same account in Yousician and Piano by Yousician as long as you log in with your account email address/username and password.
If you have a single instrument premium subscription, you can access the other instruments, but your playing time is limited.