When you kill enemies you can sometime find loot items with the same name as your skills.
These items can be browsed in the Skills menu when you click on one of the skills you have.
The collection will be showed on the right menu plane. You can choose a specific item there to replace the current one, You may see that there is color schema similar to other games.
Gray - regular one
Green - not regular
Blue - less common
Cyan - rare


The higher the tier the more additional minor upgrades you can apply.
Each skill icon has a + number in the corner which tells you what is the level that corresponds to number of updates that can be applied right away

When you select a skill in the Skills menu, you can switch the skills items to open new abilities for minor updates.
For example, if you have a gray one as a current skill and switch to a green one, you may fin there two options for minor upgrades. The blue one will allow you to apply three upgrades. At the bottom of the menu, you will see the XP status of the current skill item.


When the item is chosen and placed in the current skill slot, you will gain XP for this skill while using it and killing enemies. So it is better to always check what skill items you have and switch to the ones you desire, so you can upgrade skills as by gaining XP. Once you have a skill level up, you can put the skill point into one of the upgrades. The number of skill points will be shown with white bars at the a bottom of the description, near to the skill level number. Gray ones show already spent points.