[{"Post":"this is a post","_id":ObjectId("630f3c32c1a580642a9ff4a0"),"iframe":"\"https:/www.youtube.com/embed/RzVvThhjAKw\"","slug":"this-is-a-title","title":"This is a title"}]

db.collection.update({"_id":ObjectId("630f3c32c1a580642a9ff4a0")},[{"$set":{"iframe":{"$replaceAll":{"input":"$iframe",/** find every double quote ...*/"find":"\"",/** ... and replace it with nothing*/"replacement":""}}}}])