Hi could you please let me know how to re sync 1 month old broken DAG.

we moved service to secondary site (site b) after major out edge at primary site( site a) , and wan to keep service live at secondary  site (site b) as primary on which servers are running and primary site ( site a) will act as secondary .

now concern is if add primary site to DAG will it sync form secondary (site b) to primary site i.e ( site a) or how it will sync so data wont loss.

Hi,

According to your description, I understand that you want to re-add original DAG member to DAG, then switch active side to DR site.
If I misunderstand your concern, please do not hesitate to let me know.

Are the old servers still in the DAG?
If so, please remove the old database copy first and add DAG member again. Then run below command with both DAG members to double check the status of DAG:
Get-MailboxDatabaseCopyStatus Server MBX1 | FL MailboxServer,*database*,Status

For your second question, we can deploy server switch or datacenter switch. More details please refer to:
https://technet.microsoft.com/en-us/library/dd298067(v=exchg.150).aspx

Thanks