Ah! Yes! Another Apple user!
When you say resolution, I assume you mean changing the amount of pixels in a photo (e.g. changing a 5184 x 3456 pixel image to 1500×1000). Photos—really a more powerful editor than it lets on to be (give Apple a win, as I don’t think Windows 10 has anything similar built-in)—actually has something to do this. I’m using macOS Sierra, but I’m pretty sure this is going to be the same all the way back into Yosemite and forward into Mojave.
Select the image(s) you want to downsize, and then click File —> Export > Export 1 Photo…. After that, you’ll get a little menu. You can set your photo kind (JPEG, PNG, or TIFF), quality, color profile, and size. You can use their “Large, Medium, Small” presets, or you can click “Custom.” I’d use the Max Dimension setting (it shows you what the particular setting is measuring for portrait or landscape orientation), and then set your maximum dimension in pixels. This resizes the image (and then you can post it here!). But you are right, you can’t change the actual “resolution” (pixels per inch) in Photos, I don’t believe. You can only resize it.
However, since you’re on a Mac, you can also do this in Preview; it actually gives you control over Resolution. Open your image in Preview. Then click Tools —> Adjust Size… from their, you select “custom” and then can adjust the width and length of the image in pixels, inches, centimeters, etc. and you can adjust the resolution (pixels per inch or pixels per centimeter).
Hope this helps!