Database
[{_id:ObjectId("5a934e000102030405000000"),CanonicalWebLink:"test/mordu/episodes/try"}]
Query
db.collection.update({CanonicalWebLink:{$regex:"/mordu/episodes/"}},[{$set:{CanonicalWebLink:{$concat:[{$arrayElemAt:[{$split:["$CanonicalWebLink","/mordu/episodes/"]},0]},"/mordu/webseries/",{$arrayElemAt:[{$split:["$CanonicalWebLink","/mordu/episodes/"]},1]}]}}}],{multi:true})