If you want the same bell-type for all segments (warmup, main session, and cooldown):
1. Go to the Timer Settings page (by tapping the Settings icon in the top right corner of the Timer view).
2. From the bottom of Timer Settings, tap the selected Default Bell and set the bell type you prefer from the list of bells.
If you want different bells for each segment type, you can see how to configure the segment bells & intervals here.