How do I reverse a photo in iPhoto?

Starting with the version of Apple's Photos app that came with El Capitan (macOS 10.11), Photos began supporting 3rd party plugins.

For digitizing 35mm print negatives, I use the "RAW Power" application/plugin (currently $15 on the Apple Mac Store). From within the Photos app, you can edit any image in your Photos library (including all camera RAW files). There's a curves preset within the plugin called "Invert" which will invert the image colors. This won't completely compensate for the brown (well, cyan after inversion) hue of print negatives, but you can adjust the red, green and blue curves to compensate for that then save your curve tweaks as a new preset.

What do you mean by reverse? The layout or reverse/invert the color, i.e. from positive to negative image

If you want to flip the image like this

It can't be done in iPhoto. You would need to use an external editor from within iPhoto that can flip images.

It can be done in the cropping pane of Photos editing section.

If you mean to invert/reverse the color scheme like this

It can't be done in either app. Both would need to use an external image editor that can invert an image from positive to negative.