how do I export an HD version of my apple motion project

The best option is to create (or use) a Preset (just to the left of Destinations in Preferences).

Chances are, you already have a 1080 HD preset, check it to make sure it has the Frame Rate you want and that the pixels are square.

Once you have an appropriate Preset, you can select the Project ("layer"), select the Inspector tab then the Properties tab, then select the Preset from the drop down.

Shown is a Preset named Custom and the new size of 1280x720.

Now, when you go to share/export, the size of the exported video will be the new project size.

The drawback:

All your content will need to be resized to fit the new boundaries.

I recommend: duplicating the project first to make a copy so you don't mess up the original.

Place ALL your content into a single parent group and scale the new Group to fit the new size.

You can also create a custom preset in Compressor and use that as a preset in Motion.