To approve a post you have scheduled, simply to to your main dashboard. Here you will see the 'Select and Approve' section at the top of the page where you can view and approve the posts you have scheduled over the next month.
Just click select on the one you want to approve, choose the post you'd like to use from the options, edit the details as necessary and then click approve, super simple. You can also edit the time you would like the post to go out, if you need to.
Once you're happy, click approve and this will move to your scheduled tab, ready to go at your selected date and time.