Update Multiple Rows in Entity Framework from a list of ids

var idList=new int[]{1, 2, 3, 4};
var friendsToUpdate = await Context.Friends.Where(f => 
    idList.Contains(f.Id).ToListAsync();

foreach(var item in previousEReceipts)
{
  item.msgSentBy = "1234";
}

You can use foreach to update each element that meets your condition.

Here is an example in a more generic way:

var itemsToUpdate = await Context.friends.Where(f => f.Id == <someCondition>).ToListAsync();

foreach(var item in itemsToUpdate)
{
   item.property = updatedValue;
}
Context.SaveChanges()

In general you will most probably use async methods with await for db queries.

something like below

var idList=new int[]{1, 2, 3, 4};
using (var db=new SomeDatabaseContext())
{
    var friends= db.Friends.Where(f=>idList.Contains(f.ID)).ToList();
    friends.ForEach(a=>a.msgSentBy='1234');
    db.SaveChanges();
}

UPDATE:

you can update multiple fields as below

friends.ForEach(a =>
                      {
                         a.property1 = value1;
                         a.property2 = value2;
                      });