Mirror the selected embroidery or block of stitches vertically or horizontally using the flip handles on the selection box.
There are no flip handles for multiple selections or groups; if desired, use Combine in order to mirror. There are no flip handles for Monograms; if desired fix as embroidery, then mirror.
Move the pointer over the Flip Horizontal handle 
and it becomes the horizontal double-arrow pointer 
. Click the Flip Horizontal handle to mirror an embroidery or selected block horizontally.
Move the pointer over the Flip Vertical handle 
and it becomes the vertical double-arrow pointer 
. Click the Flip Vertical handle to mirror an embroidery or selected block vertically.