Firestore - How to decrement integer value atomically in database?

To decrement the value in a field, do a negative increment. So:

FieldValue.increment(-50)

Make sure you're not accessing FieldValue from a Firestore instance. Eg. (in node.js with admin sdk)

admin.firestore().FieldValue.increment(-50) // Won't work
admin.firestore.FieldValue.increment(-50) // Will work

This did it for me.