Hi there, verarsalha. Thanks for reaching out and welcome to the Community. Allow me to clarify alarms and how they work with Alexa and Sonos. The alarms created in the Sonos app are accessible only from Sonos. When asking Alexa anything to do with alarms, the command stays within Amazon and pertains to alarms on Alexa.
If you ask Alexa to set an alarm, the alarm lives on Amazon and Alexa.
If you create your alarm on Sonos, it only lives on Sonos and cannot be edited via Alexa.
You can, however, use Alexa to control playback on your Sonos devices.
This means if you were to have a Sonos One in your Bedroom and had an alarm set for 7 am in the Sonos app, at 7 am you could ask Alexa to "Stop in the Bedroom" and that should stop the playback of your alarm. Then at that point, you could control the player as you normally would with Alexa. Keep in mind, there was no use of the word "alarm" when asking Alexa to control the playback in the Bedroom.
Hopefully, you find this helpful.