Chrome/Firefox/Safari: Boomerang, a popular browser extension for Gmail users, schedules your messages to be sent at a later time or reappear in your inbox at some point in the future. In the original version, you had to use a "manage page" to edit and scheduled items, but now you can do that directly from the message itself in Gmail.
The latest update makes this very easy. To edit a scheduled message, all you have to do is open it up in Gmail and click the edit button. Then, change the date and time to what you want. To use this feature now, just update your extension. If you're new to Boomerang, you can find the latest version here.
Manage Scheduled Messages Inside Gmail [Baydin Blog]