Since you have already taken admission in emacet now,it is only possible if you cancel your admission in emacet and then apply for dost and this should be done if you are sure that you will be able to get a seat in dost which means you must have a good rank, if you cancel your admission in emacet , your seat would be given to another candidate and you won't be able to get the same seat in btech , so, be careful if you are planning to cancel the seat , be sure that you will gain admission in dost.