How would I save a value for all users in the data store for example:
The value is 10 when a player joins the value changes to 20 when another player joins.
Then all the players who have player before and arent currently playing in-game how would I change their value to 20 without them joining the game?
Youd have to set default values for whatever you want the player to have
In that case, you should store every player that joins the game, then loop through the table and do what you need to do.
I suggest saving their userid, as anyone can change their name.
Or, if you know who you want to change it for already, just use their userid.