[{"info":[{"field1":"String1","field2":"String2"},{"field1":"String1","field2":"String_2"}],"var":"x"}]

db.collection.update({"info.field1":"String1"},[{"$set":{"info":{"$map":{"input":"$info","in":{"$cond":[{"$eq":["$$this.field1","String1"]},{$mergeObjects:["$$this",{"field1":"STRING_NEW"}]},"$$this"]}}}}}],{multi:true})