How can I reverse a photo. I am trying to create a collage and I want the subject of my photo to be looking in the opposite direction. Can I flip a photo to do this.

Assuming you are talking about a photo in your Mac / iOS device, you can import it to Photos app, then transfer it to your desktop (on a Mac), and open it with Preview app (Control + Click - Open With: Preview), the select Tools from the upper menu, select Flip Horizontal, Save, and drag it back to your Photo library.

In case you are using an iPhone or other iOS device, I suggest downloading the Photoshop Express app (it's free), tap Edit at the top of the screen, select the photo you want to edit, tap the crop icon at the lower right side of the screen, tap rotate and select Flip Horizontal.

In the Photos.app on your Mac you can select the thumbnail of the photo and use the command "Image > Flip Horizontal" to create a mirror image. If you just want a second version for the collage and keep the original version as it is, duplicate the thumbnail before you mirror it with "Image > Duplicate". Then you can discard the mirror image after you exported it for your collage.