[{account_id:"ORG1",progress:[{week:1,goal:5000,raised:2400},{week:2,goal:5100,raised:1000}]},{account_id:"ORG2",progress:[{week:1,goal:5000,raised:2400},{week:2,goal:5100,raised:1000}]}]

db.collection.update({account_id:"ORG1"},[{$set:{lastItem:{$last:"$progress"},rest:{$slice:["$progress",0,{$subtract:[{$size:"$progress"},1]}]}}},{$set:{"lastItem.raised":{$add:["$lastItem.raised",77]}}},{$set:{progress:{$concatArrays:["$rest",["$lastItem"]]},lastItem:"$$REMOVE",rest:"$$REMOVE"}}])