With additional CSS, you could float the cancel button to the right.
.modal-footer .btn-default {
  float:right
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" media="screen" charset="utf-8">
<div class="modal-footer">
  <button type="button" class="btn btn-default">Cancel</button>
  <button type="button" class="btn btn-primary" data-dismiss="modal">Delete</button>
</div>
 
 
or
You can use one of the helper classes on the first button --- pull-right.
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" media="screen" charset="utf-8">
<div class="modal-footer">
  <button type="button" class="btn btn-default pull-right">Cancel</button>
  <button type="button" class="btn btn-primary" data-dismiss="modal">Delete</button>
</div>
 
 
You may additionally need to add a clearfix depending on other content in the modal.