The default way of increasing your Stat is by leveling up your character. Upon leveling up, random Stats will increase by 1. Some characters have innate affinity towards certain stats, which will affect their eventual growth or class promotion.
This is particularly important to Byleth since he needs to raise specific stats in order to recruit students from other houses. By default, Byleth starts off with the talent for Sword, Brawl, and Authority skills. As such, it will be easy for him/her to have stat growth on Str, Dex, Spd, and Cha.
If you want to Recruit characters that require high Mag Stat (e.g. Mercedes, Annette, Lindhart), you’ll need to switch to a spellcaster class as early as possible then increase your levels several times to develop your Mag stat.