Hi Poongunfran,
Welcome to our forum.
We could update SFB server 2015 RTM to the latest directly. We update SFB server 2015 FE firstly, then SFB backend database, which is SQL server, run the command “Install-CsDatabase -Update
-ConfiguredDatabases -SqlServerFqdn <SQL Server FQDN>”, we could refer to the following link to update SFB server 2015 step by step:
https://support.microsoft.com/en-us/kb/3061064
The latest CU of SFB server 2015:
https://blogs.technet.microsoft.com/uclobby/2015/06/22/skype-for-business-2015-cumulative-update-list/
If there are any questions or issues, please be free to let me know.
Best Regards,
Jim Xu
TechNet Community Support
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