Add this using to your code:
using BLToolkit.Data.Linq
Now you can use Update<T>
extension method in your DbManager
. This method allows you to do batch update by passing collection of entities as a parameter.
In your case use it like this:
using(var db = new DbManager)
{
foreach(var user in users)
user.Username = user.Username + "!"; //update each user in collection first
db.Update<User>(users); //just one database call for batch update
}