How to change the player's Max Health?

For limiting the hearts, you would want to create a player-persistent number variable, (you can do this in the 'variables' tab of the workspace), and then run a procedure with the 'on player update tick' global trigger that checks if the player's health is greater than the variable, and, if it is, sets it to the current limit. You could then change the variable to change the limit on the player's hearts.

Adding more health is a bit trickier. Depending on how you want this to work, you could just give the player absorption with varying levels. (Every level of absorption grants the player an additional two hearts.) If you want it to better resemble normal hearts though, you might have to design your own system, or do something with custom code. (I know whatever mod they're using on Secret Life allows this, so it might be worth looking into that.)