Use Apple Watch to breathe mindfully
The Breathe app 
on your Apple Watch encourages you to set aside a few minutes a day to relax and focus on your breathing.
Start a breathe session
Open the Breathe app
on your Apple Watch.Tap Start, inhale slowly as the animation grows, then exhale as it shrinks.
Set the duration of a breathe session
Open the Breathe app
on your Apple Watch.Turn the Digital Crown to increase the duration.
You can choose a time between one and five minutes.
To set this as your default duration, open the Apple Watch app on your iPhone.
Tap My Watch, tap Breathe, then turn on Use Previous Duration.
Adjust breathe settings
You can change how frequently you get breathe reminders, mute breathe reminders for the day, and change your breathe rate.
Open the Apple Watch app on your iPhone, tap My Watch, tap Breathe, then do the following:
Set breathe reminders: Tap Breathe Reminders.
Mute breathe reminders: Turn on “Mute for today.”
Change your breathing rate: Tap Breathe Rate to change the number of breaths per minute.
See your heart rate during breathe sessions
Open the Health app on your iPhone.
Tap Health Data, then tap Heart.
Swipe left on heart rate to see Breathe.
Use the Breathe watch face
Add the Breathe watch face to get quick access to breathe sessions.
With the current watch face showing, firmly press the display.
Swipe left all the way to the end, then tap the New button (+).
Turn the Digital Crown to select Breathe, then tap to add it.
Tap the watch face to begin a breathe session.