There are currently four upgradeable skills in dungeons: health, regeneration, critical chance, and critical damage.
Each skill is upgradeable up to level 20 through the Skill Master (/warps).
To view your skill trees, use /skills.
This menu allows you to view the current level of all your skills as well as what each upgrade does and what the max level is.
Throughout dungeons and crates you can find skill plumes to upgrade the above mentioned skills.
To use plumes, teleport to the Skill Master through /warps.
The NPC here will upgrade your skills for plumes and some money.
To view pricing, simply hover over each skill and read the lore.
Wild plumes are plumes that are assignable to any skill you would like.
To convert a wild plume into a specific skill plume, visit the trader in spawn (or town merchant).
Then trade your wild plumes with some cash for a skill plume in whatever skill you desire.