Once you've scheduled a post, it is not possible to reschedule it. However, you have the option to create a copy of the post and then select new ambassadors and choose a different date and time for the duplicate post.
To make a copy:
Go to Posts & Planning and click on this button.
The post will be copied.
Click the Edit button on the copied post.
Select your brand ambassadors in Step 2.
Select a date & time in Step 3.
Click on Plan this post.