Go to Planner > Schedule

After that click Calendar

If you don't see some of the posts, click the Calendar button on your top right corner.

Once you can see your desired post, go ahead and click it

Once you click, you are going to be brought to the same window where you scheduled the post, so change the date and time you want to reschedule and click Reschedule

After you are done, you should see your post on the date you clicked to reschedule.