Changing token health in combat can be a bit disruptive, so we have a quick-menu set up for On-The-Fly edits to health. First thing to do is to select the token(s) you wish to edit. You can select more than one token for AoE effects. To select multiple tokens, left-click and drag over the tokens.

Select the "Heal/Damage HP" button, then input a number you want the HP healed/damaged by. Then, by using the up or down chevrons, you can heal/damage all selected tokens by that number.

To fully kill or fully heal a selection, use the skull or the heart icon, respectively.