Double click an equipment to equip it on the hero.

The "Bag" menu displays all equipments.

The "Potion" menu displays all consumables / stacked items.

The other menus display invidual item types, weapon, shield and misc.

On the initial weapons, note that the repeated weapons have different properties - equip them to see the hero stats change!

Scroll with mouse or sliding like you would do on a touch screen.