Steps to change SGA in 12C

Hi,

I am very new to 12c.In our DB we enabled AMM .The DBA set memory parameters to 65G and mentioned SGA as 35G and PGA as 20G previously. We are having high db file sequential read and SGA starvation is observed.I was trying to set SGA to zero with alter command (following old 11g version method).But getting below  error.

SQL> alter system set sga_max_size=0 scope=spfile;

alter system set sga_max_size=0 scope=spfile

*

ERROR at line 1:

ORA-65040: operation not allowed from within a pluggable database

I wanted to see SPFILE contents too as below.Again same error.