How many clients do you have in your network?
if the count is less.
Install SEPM 12.1 on a new server
follow this document to update communication settings.
http://www.symantec.com/business/support/index?page=content&id=TECH199124
You can then use autoupgrade feature to update all your clients to 12.1
check the autoupgrade procedure here, covers installaion as well.
http://www.symantec.com/business/support/index?page=content&id=TECH197426
once you run setup.exe it will ask to run symantec endpoint, if you click on that, it will give you option to install manager.