Go to the "Levels" tab on a skill matrix.

Click on "Edit basic levels".

Important! You can change skill levels only if there are no active reviews on a skill matrix at the moment. (Read more here)

You'll find the editor's window. 

Here you can add a new skill level. 

Or make changes to the existing levels:

Click "Save changes" when it's ready. 

If the current skill levels have descriptions, you will have to match them to the old ones. Select the level or "no match" option if you have created a new level with no description.

Save all changes.

Press "Cancel" if you want to continue editing or leave it unsaved.