How do I change a color photo to sepia in Photos for Mac?

Photos does not have a sepia filter. You can try to mimick it by reducing the saturation in the Color adjustment, until the image is nearly black-and-white, then shift the Warmth in the White balance adjustment. The "Transfer" filter will give an effect similar to Sepia.

Or try an editing extension like "Color Filters" or FX Photo Studio CK.