[{"_id":1,"_deleted":false,"customFields":[{"fieldName":"sapID","value":""},{"fieldName":"salesTerritory","value":""},{"fieldName":"clientType","value":"Corporate"}]}]

db.collection.aggregate([{$set:{customFields:{$map:{input:"$customFields",as:"c",in:{$cond:{if:{"$eq":["$$c.fieldName","clientType"]},then:{value:"$$c.value"},else:"$$c"}}}}}}])