For more information regarding the ALTER ROLE command, refer to the Postgresql documentation.
In addition, you can use the ALTER USER command, which is an alias for ALTER ROLE:
Note, if you change the password for the APM Database User, not to be confused with Postgresql Administrative User, this information will also need to be updated on each of the Enterprise Managers. Otherwise they will no longer run. To do this, open <EM Home>\config\ tess-db-cfg.xml and update the hibernate.connection.password with the new password, in plain text. Also set plainTextPasswords to true, so that the EM password is encrypted on EM restart, and then restart the EM.
As always, contact CA Support if you have further questions.