Database
[{"userId":"Ldu9N8DTIjfUApMlLUhuwzxZszt1","userName":"Fake Star","userChats":[{"chatId":"Ldu9N8DTIjfUApMlLUhuwzxZszt1Jaz5seHjL0Nct0IdSgixRW9wM8s2","sender":"Ldu9N8DTIjfUApMlLUhuwzxZszt1","receiver":"Jaz5seHjL0Nct0IdSgixRW9wM8s2","receiverPic":"receiverPic","lastMessage":"lastMessagelastMessagelastMessage","timeStamp":100},{"chatId":"Ldu9N8DTIjfUApMlLUhuwzxZszt1Jaz5seHjL0Nct0IdSgixRW9wM8s2","sender":"Ldu9N8DTIjfUApMlLUhuwzxZszt1","receiver":"Jaz5seHjL0Nct0IdSgixRW9wM8s2","receiverPic":"receiverPic","lastMessage":"lastMessagelastMessagelastMessage","timeStamp":1000}]}]
Query
db.collection.update({},{"$set":{"userChats.0.timeStamp":1000}})