Database
[{"_id":1,"region":"Midwest","dob":"1989-07-21T05:00:00.000","paymentOption":{"_id":2,"anotherProp":3,"coverage":{"status":"active","coverageEnd":"2020-07-31T00:00:00.000","coverageStart":"2018-08-01T00:00:00.000"}}}]
Query
db.collection.aggregate([{$addFields:{"coverage":"$paymentOption.coverage"}},{$project:{paymentOption:0}}])