[{"_id":1,materials:{"m1":{inventory:[{price:100,amount:65}]}}}]

db.collection.update({_id:1},[{"$addFields":{"materials.m1.inventory":{$concatArrays:[[{price:{$getField:{field:"price",input:{$first:"$materials.m1.inventory"}}},amount:100}],{$slice:["$materials.m1.inventory",1,{$size:"$materials.m1.inventory"}]}]}}}])