Changing SGA & PGA memory for Oracle

The following can give good info on target memory advice after your system has been running for some time. It is cleared during restart
 select * from V$MEMORY_TARGET_ADVICE ;

BACKUP your SPFILE before doing any radical changes.
Example location ~ D:\oracle\product\12.1.0\dbhome_1\database

To set pga to 1 gb and sga to 3 g, enter the following commands:

ALTER SYSTEM SET pga_aggregate_target=1G SCOPE = SPFILE;
ALTER SYSTEM SET sga_target=3G SCOPE = SPFILE;

You will need to restart your Oracle database.

And, Bob's your uncle, you should have your PGA and SGA set to the new target values.