How to save player1's stats separate from player2's i.e. Health, Damage, Etc..

I have a game that I want to eventually be multiplayer so how can I have player1 and player2 choose characters with different base stats and upon leveling up they choose to upgrade their Health,Damage, or Speed.

I have the character selection down I just need help on how to save the stats for the players without their upgrades affecting the other players on the same character.

You can serialize the player data and store it in PlayerPrefs. Check this Unity tutorial on how to save any data into PlayerPrefs.