Just to add to CKoenig's response. His answer will work as long as the class you're dealing with is a reference type (like a class). If the custom object were a struct, this is a value type, and the results of .FirstOrDefault will give you a local copy of that, which will mean it won't persist back to the collection, as this example shows:
struct MyStruct
{
public int TheValue { get; set; }
}
Test code:
List<MyStruct> coll = new List<MyStruct> {
new MyStruct {TheValue = 10},
new MyStruct {TheValue = 1},
new MyStruct {TheValue = 145},
};
var found = coll.FirstOrDefault(c => c.TheValue == 1);
found.TheValue = 12;
foreach (var myStruct in coll)
{
Console.WriteLine(myStruct.TheValue);
}
Console.ReadLine();
The output is 10,1,145
Change the struct to a class and the output is 10,12,145
HTH