There are several Skill Enchanting options at your disposal: General Skill Enchant, Blessed Skill Enchant, Skill Un-Enchanting and Skill Route Change.
General Skill Enchanting This method of enchanting requires 1 [Giant's Codex] and a certain amount of SP and adena to complete. If the enchant attempt fails, then the skill enchant level is returned to zero.
Blessed Skill Enchanting
This is a safer way to enchant a skill. This method requires 1 Giant's Codex: Mastery and 5 times the SP and adena of a general skill enchant. But, should you fail to enchant the skill, the value is not reset to zero. This option is good for higher level enchants.
Skill Un-Enchanting
This option refunds the user with some of the SP originally spent on the skill. This method requires 1 Giant's Codex: Oblivion, and takes the skill enchant down one level at a time. 80% of the original SP spent on the skill will be refunded to you. And if used when the skill is at level 1, the skill will be reset to zero.
Skill Route Change
This feature changes the type of enchant applied to the skill. In addition to the Giant's Codex: Discipline, the SP and adena required to change the route is 20% of the SP and adena needed for the next level of the enchant. When used, there is a chance that the enchant level may stay the same or may be reduced by 1-3 levels.