In Brink you are able to Respec your character—meaning that when you press the button, all of your Abilities will be unlearned, and you will able to distribute your Level Credits again. The button is labelled "Sell All" and is found in the Character Customization menu.
Beware that choosing to do this costs the player one level, and the effects that one would expect with the loss of a level, e.g. a character dropping to Level 19 from 20 will lose access to the outfits unlocked at Level 20, as well as the ability to spend Level Credits on Rank 5 class abilities.
Your XP will be reset to the starting value of the previous level, so you have to regain all XP of this level to reach the original state again. Upon reaching the original level, you will also be rewarded with the missing Level Credit. Respeccing does not have any permanent negative effect.
After the Agents of Change DLC respeccing on levels above 20 is advised shortly after gaining a new level. Due to the total XP reset on the base value of the previous level, you could possibly lose XP worth almost two levels. With the larger XP gap between each level this can be a fairly large amount.