Hi Anil Kondalkar,
I would suggest following path:
1) Take a complete backup of existing SEP 11.x prior to upgrade
http://www.symantec.com/business/support/index?page=content&id=TECH102333&locale=en_US
2) Check for sufficient disk space because SEPM 12.1 upgrade installer requires three times the current database size (sem5.db) if upgrading from pre-12.1 SEPM with the embedded database only.
Reference : http://www.symantec.com/docs/TECH162730
3) Download latest version i.e SEP 12.1 RU1 from fileconect.
4) Go through following article to know upgrade process overview
https://www-secure.symantec.com/connect/articles/sep-11x-sep-121-upgrade-process-graphical-overview
Video also now available for same
https://www-secure.symantec.com/connect/videos/upgrade-sep-121
Upgrade process might take upto 30 minutes to upgrade existing SEPM in this period SEP client won't be able to connect SEPM. After SEPM ugprade restart is not mandatory however it would be great if you could do so.
With buffer time you can schedule 1 hour downtime to upgrade existing SEPM. Downtime is required only if scheduling reboot after upgrade.
After successful SEPM upgrade you can upgrade existing SEP clients to latest version by using Auto upgrade option
http://www.symantec.com/docs/TECH96789
Check this article as well.
https://www-secure.symantec.com/connect/articles/sepm-121-auto-upgrade
In this way you can complete upgrade for SEPM and SEP clients.
I hope it will help you.