[{_id:1,sections:[{Title:"hello world",Title2:""},{Title:"goodbye",Title2:""}]},{_id:2},]

db.collection.update({sections:{$exists:true}},[{$addFields:{"sections":{$map:{input:"$sections",as:"s",in:{"Title":"$$s.Title","Title2":"$$s.Title"}}}}}],{multi:true})/** "sections.Title2": "$sections.Title",*/