Visit the Skill Mentor NPC at Fleet or your Faction's capital.
^This. The skill mentor (on the fleets) can be found on one of the far sides of the Combat Section. As for on the faction capital, I cannot remember where they are other than for Imps the skill mentor is at Kaas City and for Republic at the Senate Tower.
Likewise, you can buy a legacy unlock (called Field Respec) which will allow you to change your skill tree anytime you want.