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

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