Syntaxis UPDATE JOB in Heroku Scheduler

I want to update a table of my DB MySql through a JOB. I looked for several ej but none works.

Something as simple as this ....

UPDATE asistencias_cursos 
SET estado_curso = 'C' 
WHERE estado_curso = 'G;

I tried this, but it was not ...

require "sequel" DB = Sequel.connect ENV["DATABASE_URL"] 
puts "UPDATE asistencias_cursos..." 
DB["UPDATE asistencias_cursos SET estado_curso = 'C' WHERE estado_curso = 'G'"] puts "done."

I would appreciate it if someone has an example of a simple UPDATE JOB.

Thank you!