Who Can Alter A Role

The ALTER ROLE statement can be executed by:

Administrators
Users with the ALTER ANY ROLE privilege, with the following caveats
- Setting or dropping system privileges also requires the system privilege CREATE_PRIVILEGED_ROLES
- Setting or dropping auto admin mapping also requires the system privilege CHANGE_MAPPING_RULES

Drop all system privileges from a role named SELECT_ALL

ALTER ROLE SELLERS
  DROP SYSTEM PRIVILEGES;

Grant a role SELECT_ALL the system privilege to select from any selectable object

ALTER ROLE SELECT_ALL
  SET SYSTEM PRIVILEGES TO SELECT_ANY_OBJECT_IN_DATABASE;