Choose single player mode and select an open space and no enemies at the start. Press E to enter Edit mode. Press F3 to spawn a new entity (default is shotgun shells). Press F4 to change the entity to a health boost upgrade. Repeat those steps to spawn as many health boosts as desired. Each upgrade gives you an additional 5 HP to your maximum, and remains after death. Note: While in Edit mode you are fly and see through walls. However, enemies will respawn once you enter and exit editing mode.