How to change SGA size

In sqlplus, execute

ALTER SYSTEM SET sga_max_size = 4G SCOPE=spfile
shutdown immediate
startup

This is only for starting Oracle with spfile. If you execute
ALTER SYSTEM SET sga_max_size = 4G SCOPE=memory or
ALTER SYSTEM SET sga_max_size = 4G SCOPE=both

you will get
ORA-02095: specified initialization parameter cannot be modifie.

If oracle is automatic shared memory management (ASSM) mode, also execute
ALTER SYSTEM SET sga_target = 4G