There are multiple ways a character can gain stat points. There are the ways.
1. Leveling up : A person can gain stat points by leveling up. Each level up gives 4 stat points. First level up requires 10 exp. 2nd level up requires 20 exp. And so on.
2. Training : A person can also gain stat points by training. For example doing physical training increases strength, agility, defense. Mental training increases perception, intelligence etc.
3. Natural : A person can also passively become stronger. This happens of course because of puberty.
4. Magic Items : There exists magical items that can boost a persons power. Some through transforming the body and some directly.
5. Acheivements : Acheivements can reward stat points. Some reward free use stat points. Some directly increase one stat.
6. Teaching or Mentoring : This only applies to mental stats. Being taught by a teacher or mentor can increase mental stats.
7. Consumables : Certain consumables can temporarily boost a persons power.
8. Blessings or Curses : Magical blessings or curses can temporarily strenghten or weaken a person or creature.
9. Social interaction : Succesfully persuading a person can increase a persons charisma. But not succeding can still increase charisma depending on how difficult persuading the other character is.