[{"_id":ObjectId("5ebeacb2c4ec53c0621426a6"),"abc":1,"docs":[{"a":"a","b":"b"},{"a":"a","b":"b"},{"a":"a","b":"b"},{"a":"abc","b":"b"}]},{"_id":ObjectId("5ebeacb9c4ec53c062142746"),"abc":2,"docs":[{"a":"a","b":"b"},{"a":"a","b":"b"},{"a":"a","b":"b"}]}]

db.collection.aggregate([{$match:{abc:1}},{$set:{docs:{$concatArrays:[[{$arrayElemAt:["$docs",0]}],{$map:{input:{$slice:["$docs",1,{$size:"$docs"}]},in:{$cond:[{$eq:["$$this.a","a"]},{$mergeObjects:["$$this",{"b":"c"}]},"$$this"]}}}]}}}])