Measurement units

You can't. Garmin only uses Calories, which are in fact metric too (just not SI).

Calories aren’t metric! Energy is measured in kj in the SI system -which is the metric system. 

They are metric - one calorie is by definition the amount of energy (heat) needed to raise the temperature of one milliliter of water by one degree Celsius (or one kelvin) - all metric units. They are just pre-SI, and are not included in the official SI set.

Regardless, Garmin only uses Calories (kcal), not joules, for both metric and statute systems.

BTW, the metric system was established during the French Revolution in 1799 (Mr. Joule was not born yet, at all, in that time). SI was founded in 1875, and the energy unit 'joule' was introduced a decade later, in 1889. Calorie was officially deprecated as an SI unit only in 1948. It does not mean yet, it is not a metric-based unit. There are plenty of other metric units that are deprecated in SI.

The problem is most countries label their  food using kj not calories even if it used to be included in SI units. I think the US and Liberia are the only countries that use the old system