Learnt that H1B alloted. Now, I resigned to join dream job.How to transfer the alloted to new employer ?

I have completed my Masters and joined a reputed company and not had job satisfaction. In view of this, I decided to change my job and got offer from another reputed company ( dream job) and intend to join during June 2017.Now, I have been relieved two days back after my resignation and came to know H1B allotted and Receipt No mailed to Employer but not communicated to me as on now.

Advise me how to get it transferred or employer can withdraw ?

Why would you resign, when the join date of the job is in June? Once you know that H1B has been applied, you should have waited to know the result of it before making a decision, as getting into the lottery has been difficult lately. You have receive the receipt number only, the petition needs to be approved, in order to be transferred.

To transfer your petition you need to have a copy of the approval notice. Talk to the immigration lawyer of the new company you are going to join, to see if just the receipt number should help in future, in case the petition gets approved by June.