Why can't I see my Equipment in the Upgrade menu?
The Upgrade menu does not contain Equipment for your regiment, so you cannot upgrade items here. Here, you can only improve your regiments' abilities by applying upgrade cards.
If you want to upgrade one of these, go to the Forge menu and tap on one of the items that can be upgraded. Those are indicated by a green + icon. There, you can apply upgrades and reforge your items to higher rarities.