Each time you gain a level, you gain 1 additional Hit Die. After rolling that Hit Die, you may choose whether you want to keep the roll or reroll. You may reroll as many times as you want, as long as the new roll is not equal to or below the previous roll. You must keep the latest roll. Once you’ve rolled your Hit Die, add your Constitution modifier to the roll, and add the total to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry (before rolling), which is the average result of the die roll (rounded up).