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Alter user in Oracle SQL

Alter user <user> quota

alter user <user_name> quota unlimited on <tablespace_name>;
alter user <user_name> quota 1M on <tablespace_name>;
Sets the quota for a user on a tablespace.

Alter user <user> identified by NEW_PASSWORD

Sets a new password for a user. Alternatively, the SQL*Plus command password can be used.

Alter user <user> connect through <proxy_user>

See this link.

Alter user <user> profile

This command allows to define a profile for a user:
alter user <user_name> profile change_password_profile