You can update your clients using autoupgrade feature. Apply auto upgrade feature on all group one by one except Server Group
https://www-secure.symantec.com/connect/videos/upgrade-clients-sep-121-using-auto-upgrade-feature
http://www.symantec.com/connect/articles/upgrade-clients-sep-121-auto-upgrade-feature
For Server Group you can install the client package manually.
For remote client group you can use Push Deployment Wizard
Overview of Push Deployment Wizard in Symantec Endpoint Protection 12.1
https://www-secure.symantec.com/connect/articles/overview-push-deployment-wizard-symantec-endpoint-protection-121