[{test:"test1",Attach:[{Type:"img",Url:"https://url.com dummy/"},{Type:"img",Url:"https://url.com/dummy/"},{Type:"img",Url:"https://url.com dummy/"},{Type:"img",Url:"https://url.com/dummy/"}]}]

db.collection.aggregate([{$match:{/** find documents with space inside*/"Attach.Url":{$regex:" "}}},{"$addFields":{"Attach":{"$map":{"input":"$Attach","as":"a","in":{Type:"$$a.Type",Url:{/** replace space by slash*/"$replaceAll":{"input":"$$a.Url","find":" ","replacement":"/"}}}}}}},{/** merge back to original collection*/"$merge":{"into":"collection","on":"_id","whenMatched":"replace"}}])