In this example, the SGA will be resized to 6GB and the PGA to 2GB.

First, change the SPFile:

ALTER SYSTEM SET pga_aggregate_target = 2G SCOPE=SPFILE;
ALTER SYSTEM SET sga_target = 6G SCOPE=SPFILE;

Then, (optionally) export the SPFile to a PFile:

CREATE PFILE FROM SPFILE;

Finally, reboot the instance to activate the change:

SHUTDOWN IMMEDIATE;
STARTUP;