Change value of multidimensional ArrayList

As I've stated in my comment, just iterate through the list to find the object. If you're going to do this a lot, consider using a map.

for (ValidateUser user : personer)
    if (user.getName().equals("admin"))
        user.setNumber(someNumber);

First, note that this is not a multidimensional array, is just a list that holds elements of ValidateUser class object references. Second, you need to access to the element before updating it. You have several ways to accomplish this:

1. Implement the equals and hashCode methods in your ValidateUser class, then just retrieve the object from your List:

   ValidateUser adminUser = personer.get(new ValidateUser("admin", "", 0.8, ""));
   adminUser.set...
   

   Note: this looks ridiculous but will work (assuming your equals method only checks by the field that holds this "admin" value.

2. Navigate through the array and seek for the desired element manually, then update it:

   for (ValidateUser user : personer) {
       if ("admin".equals(user.getXxx()) {
           user.set...
           break; //don't forget this!
       }
   }
   

3. Use a different data structure like a Map<String, ValidateUser> to store your data and faster retrieval:

   Map<String, ValidateUser> personerMap = new LinkedHashMap<String, ValidateUser>();
   personerMap.add("admin", new ValidateUser("admin", ...);
   //fill the map with other values...
   //if you still want a Collection<ValidateUser> personer variable
   Collection<ValidateUser> personer = personerMap.values();
   
   //now check for the desired element
   ValidateUser admin = personerMap.get("admin");
   if (admin != null) {
       admin.set...
   }
   

By comments, your ValidateUser is an immutable object, so you cannot update its fields using setters (because there aren't). So, the best approach here is to use a ListIterator<ValidateUser> instead (not to confuse with Iterator) and replace the element by your modified object. Here's an example:

//the new immutable ValidateUser that will replace the older one...
//set the parameters as needed
ValidateUser newAdmin = new ValidateUser("admin", ...);
ListIterator<ValidateUser> listIterator = personer.listIterator();
while (listIterator.hasNext()) {
    ValidateUser validateUser = listIterator.next();
    if ("admin".equals(validateUser.getXxx()) {
        listIterator.set(newAdmin);
        break;
    }
}