Without DB Restart:

show parameter sga_target;
alter system set sga_target = 10G;

With DB Restart:

show parameter sga_max_size;
alter system set sga_max_size = 12G scope=spfile;

Memory recommendation

SQL> select * from v$memory_target_advice order by memory_size;
MEMORY_SIZE MEMORY_SIZE_FACTOR ESTD_DB_TIME ESTD_DB_TIME_FACTOR VERSION
180 .5 458 1.344 0
270 .75 367 1.0761 0
360 1 341 1 0
450 1.25 335 .9817 0
540 1.5 335 .9817 0
630 1.75 335 .9817 0
720 2 335 .9817 0