Prosthetic Tool Upgrades in Sekiro: Shadows Die Twice allows the player to improve Prosthetic Tools, and make them more deadly. Players will need to use Upgrade Materials in order to do this, and you can check out what those are on that page, but on this page will focus on just what you can upgrade them to do.
To upgrade your Prosthetic Tools, you will need to first find the tool, and gather the required materials. Progression is then done via a tree, so each upgrade requires a previous upgrade, until an ultimate path is chosen.
Note that it is impossible to fully upgrade all tools during a single playthrough as this requires 10 Lapis Lazuli, but only 6 can be acquired per playthrough (and require the player not end the game early by getting the "bad" ending).
Prosthetic Tool Upgrades
Unlocking Tier 1 reveals Tier 3 contents for tools you own, and so on. There are specific conditions to unlock each upgrade, such as having the right materials, unlocking previous upgrades, and game progression. See their individual pages for details