Hi,
Check your mailbox, you might have received upgrade notification from Symantec.In that email you would have received serial number to download SEP 12.1
If you have not received any notification please contact license department.
SEP 11.x is paper license product and SEP 12.1 is software license product.
To upgrade from SEP 11.x to SEP 12.1, first you will have to upgrade your product to SEP 12.1 trial ware.
After successful upgrade you will have to activate your license.
You can activate your license in two way.
1) By entering your serial number which generally starts with "M", after entering serial number you can click on download license file & SEPM will download license for you.
2) You can also manually import .slf file into SEPM console, if you don't have .slf file then you will have to contact license department.