[{"_id":1,"users":[{_id:12,name:"John"},{_id:13,name:"Jane"}]},{"_id":2}]

db.collection.update({_id:1},[{$set:{"users":{$map:{input:"$users",as:"users",in:{$cond:{if:{$eq:["$$users._id",12]},then:{"_id":"$$users._id","name":{$concat:["$$users.name","!"]}},else:{"_id":"$$users._id","name":"$$users.name"}}}}}}}])