Database
[{"id":"181","EID":"83","History":[{"aNum":"12324","dev":[{"type":"","room":"Office","floor":"Second Floor"},{"type":"","room":"Bedroom","floor":"n/a"},{"type":"","room":"Bedroom","floor":"First Floor"},{"type":"","room":"Bedroom","floor":"n/a"},]}]}]
Query
db.collection.update({},{$set:{"History.$[].dev.$[dev].floor":""}},{arrayFilters:[{"dev.floor":"n/a"}],multi:true})