It is not possible to change the time of a scheduled send.  Once a scheduled send has been set up, the email is added to the sending queue for the designated date and time. This time cannot be changed.  If you need to change the time, you will need to cancel the scheduled send and re-schedule it for the new desired time.

For more information on cancelling a scheduled send, see the below article:

https://support.vutu.re/hc/en-us/articles/360020557417-Cancelling-a-Scheduled-Send-and-Pausing-or-Cancelling-a-Send-Now