In the app, your heart rate zones will be set to the automatic calculation by default but you can switch to manual if you wish to input your heart rate zones yourself. 

To view and edit your heart rate zones, tap on your profile photo on the Home tab then the cog in the top right corner. You can then tap Heart Rate Zones. 

Once in this screen, you can switch between manual and automatic and view the calculation used for the automatic values by tapping on the "i" symbol.

You can also access the Heart Rate Zones settings through the Swim Details screen underneath the heart rate breakdown.