Content > Approval
Hover over the post you want and click Approve/Reject to select
In the edit window, click Reject
Draft will now move back to the section Content > Drafts
Content > Approval
Hover over the post you want and click Edit to select
In the edit window, click Reject to select
Draft will now move to the section Content > Drafts
ATTENTION: it's enough if just one person rejects a post for approval.