They can be upgraded from F to A then 9 to 1.
A total of 3913 AP(Ability Points) is needed for the development of all skills. 1395 points for combat skills, 1769 points for magic skills and 749 points for passive skills.
To enable a combat or a magic skill, you must click on the skill bar, which will then turn orange, and the chance for the skill to trigger will appear at the box above the loot box. Passive skills do not need to be enabled. Combat and Magic skills can already be used at the F level without first having to invest into them.
CRITICAL NOTE: If you have multiple active attack skills turned on they each have a reduced chance of firing within a combat round compared to only one being activated, and this chance reduction gets worse the more of them you activate. Also, only one of the activated skills will ever fire within one round; no matter what your Cast Chance boost is you wouldn't be able to fire a Mirage Missle and multiple magic spells at once within a single round. Defensive skills have the same issue; you can't have both Defense and Counterattack fire at the same time within one round. These facts therefore make it far more tactically suitable to pick one good attack skill and one good defense skill and only activate/train them, so that you have a very good skill firing very often. You CAN have one attack skill and one defense skill both fire within one round.