[{Month:{"2020-12":{a:10,b:21,},"2021-01":{a:112,b:34,},"2021-03":{a:35,b:56,},"2021-02":{a:767,b:56,},"2021-04":{a:78,b:98,}}}]

db.collection.aggregate([{$replaceRoot:{"newRoot":"$Month"}}])