The other solutions here feel dirty because they mutate objects in a collection via the use of LINQ.
I would instead, put the code and the filter condition into an extension method and call that:
public static IEnumerable<Item> UnlockWhere(this IEnumerable<Item> list, Func<Item, bool> condition) {
foreach (var image in list)
if (condition(image)) {
image.IsLocked = false;
yield return image;
}
}
The keeps the immutability-concerns of LINQ intact and still produces the expected result.
The call becomes:
var unlockedItems = App.ImageListVM.Items.UnlockWhere(img => img.Category == key);
EDIT
Re-written to completely remove LINQ. Instead, this new method iterates only once and returns a new, mutated collection.