[{array:["a","b","c","d"]}]

db.collection.aggregate([{$unwind:{path:"$array",includeArrayIndex:"index",preserveNullAndEmptyArrays:true}},{$addFields:{array:{$cond:{if:{$eq:["$index",1]},then:"B",else:"$array"}}}},{$group:{_id:"$_id",array:{$push:"$array"}}}])