alter user system identified by <your_pw>;
alter user system identified by <old_pw>;
If i changed the system user password it will affect to the currently using database or not?That would depend on you and your database. It is a very bad idea to create objects in the SYS/SYSTEM/SYSAUX schemas, and if you or everybody else using your database did not do this all applications running on your database should not be affected. If you have however some things done with the system account, like for example a external script doing a expdp with the SYSTEM account then this script will fail after you change the password (but it wouldn't have worked anyway as the SYSTEM account is expired).
if i changed any other portion i have to change this password