Here's how to set JioTunes using your Android device - launch the JioSaavn app and log in with your Jio number > tap the three dots menu to the right of any song > select "JioTune & Ringtone" > Under the 'JioTunes' tab, preview your favourite JioTune > click on "Set" and you're done.
If you are on an iOS device, simply launch the JioSaavn app and log in with your Jio number > tap the three dots menu to the right of any song > select "Set JioTune" > Under the 'JioTunes' tab, preview your favourite JioTune > click on "Set JioTune" and you're done.