Hi Sasi,
For BizTalk 2006 to 2013, you need to migrate from 2006 to 2009 and then 2009 to 2013.
Step1 : Steps for migrate from 2006 to 2009:
Goto following document download page:
BizTalk Server 2009 Installation and Upgrade Guides
And download Upgrading to BizTalk Server 2009 from BizTalk Server 2006.docx in the list for details steps involved in this intermediatory migration.
Step 2: Steps for migrate from 2009 to 2013:
Goto following document download page
BizTalk Server 2013 (and 2013 R2) Installation document
And download Upgrading to BizTalk Server 2013 from BizTalk Server 2010-2009.docx in the list for details steps involved in the final leg of migration.
Regards,
M.R.Ashwin Prabhu