What the step i can perform for upgrade clients?
I have 3500 clients and 8 sub groups.
6 groups are 500+ clients
One test group and one server group.
What the gorup i can first select for upgrade or i can select all in same time?
See this thread
https://www-secure.symantec.com/connect/forums/sep-1214-mp1-agent-upgrade
Upgrade clients to SEP 12.1 using Auto upgrade feature
https://www-secure.symantec.com/connect/videos/upgrade-clients-sep-121-using-auto-upgrade-feature
How to upgrade Symantec Endpoint Protection clients using the Auto-Upgrade feature
Upgrade with autoupgrade
For server you can do it manually activity.
For clients depend on the bandwidth
Check the thread to calulate client upgrade according to bandwidth
I suggest you first you can move your client in test group and apply the autoupgrade feature here to check the success then apply on other
You can use the auto-upgrade feature within the SEPM, however, with that many clients you should consider setting the upgrade schedule out 7-14 days to not cause issues with bandwidth. If you have another option such as Altiris or SCCM, I would consider that as well.