Skill Increases
Source Player Core pg. 100
This section specifies the levels at which your character can increase their proficiency rank in a skill. At 3rd level and every 2 levels thereafter, most classes grant a skill increase, though envoys gain them earlier and more often. Your character can use a skill increase to either become trained in one skill in which they're untrained, or to become an expert in one skill in which they're already trained.
If your character is at least 7th level, they can use a skill increase to become a master of a skill in which they're already an expert. If they're at least 15th level, they can use an increase to become legendary in a skill in which they're already a master.
If your character is at least 7th level, they can use a skill increase to become a master of a skill in which they're already an expert. If they're at least 15th level, they can use an increase to become legendary in a skill in which they're already a master.