Find and replace content within a string

I literally cannot find the solution for this anywhere. Is it possible to find everything from start and end, and the content inside.

var start = "/* Start: User One */";
var end = "/* End: User One */";

var userCSS = "
/* Start: User One */ 
div.user_one {height: 700px;}
/* End: User One */

/* Start: Storm */ 
div.storm {height: 500px;}
/* End: Storm */";

I've added the start and end as I've already extracted them, but how do I do a search for the start and end, and then once I've the start and end found how do I replace everything from start to end. So ideally once I've found the following:

This is what I am looking for, and ideally be able to replace it with a variable

/* Start: User One */ 
div.user_one {height: 700px;}
/* End: User One */

I can replace it with a variable. I feel like since I'm searching for the start and end, this has to be some sort of regex solution, but my regex is heavily limited.

New content variable

/* Start: User One */ 
div.user_one {height: 500px;}
div.user_one h1 {height: 500px;}
/* End: User One */

Input

/* Start: User One */ 
div.user_one {height: 500px;}
/* End: User One */

/* Start: Storm */ 
div.storm {height: 500px;}
/* End: Storm */";

Expected output

/* Start: User One */ 
div.user_one {height: 500px;}
div.user_one h1 {height: 500px;}
/* End: User One */

/* Start: Storm */ 
div.storm {height: 500px;}
/* End: Storm */";