The materials used to craft an Upgrade Kit are different depending on which kit. The first kit, (wood to stone), uses 1 cobblestone, 2 planks, 1 string and 1 flint, where the latter uses 1 of the previous material and 2 of the latter + the previous material's upgrade kit.
When put in an anvil with an item that matches the material tier, (wood, stone, iron, gold, etc.), the player can upgrade the selected tool to a higher tier. The upgrade kit is only good for one tool upgrade, and will be destroyed after a tool has been upgraded.
- Before AbyssalCraft 1.5.5 (I think), enchantments wouldn't persist when using an upgrade kit. They do now.