There are two ways to edit a scheduled message to a guest:

See more on the second item, here.

This refers to editing the messaging rule in:

Any edits that you make to the rule here are applied to messages sent to all guests, not only some. The edits made to the content of a messaging rule will also update any existing unsent schedule messages.

Any changes made to the text, settings, or scope are saved automatically and applied right away.

If you change the scoping to remove a platform or property, then the rule will be removed from the schedule for guests who booked through that platform, or on that property.

The only caveat is if you had previously edited the scheduled message for a specific guest. Then the changes you make to the messaging rule will not apply to that guest’s message (not even to remove it). More on that below.

Automatic messages that are generated by events such as new inquiry, new reservation and new cancellation will not be updated when you make any changes to the messaging rule, even if the message has not been sent yet (this means its Timing was set to either prepare for sending manually, or delay by an amount of time).

This also means that if an event messaging rule Timing was originally set to prepare/delay and you change it to send immediately, previously generated messages will not be sent immediately.