How can I modify alert email content?

How can I modify alert email content?

Emails to guests can be customized using email templates (Settings > Email Templates). That page has a list of system-defined events and then custom ones you can create (to send with triggers). You would find the template and then create one to override the system one.

If it's a user facing alert (ie. a notification OwnerRez is sending to you as the OwnerRez user) it may not be able to be customized. Most of those are hard-coded.