So basically you gain levels when completing milestones (which also should be chosen at character creation), once you complete a milestone you gain a level.
When a PC gains a level, they can choose one of the following: • Add a +1 to their HP • Gain a new ability, trait or spell • Gain an additional momentum point.
When it comes to level for creatures, they mostly stay the same but if you would like to increase the difficulty I suggest adding additional HP or increasing the armor tier or the number of enemies or give them additional traits/abilities.
I'll try to amend the document and add that suggestion.