Assuming that you don't care which NSManagedObjectContext is used, and you just want to make some changes and save them in the background, use the following method. 90% of the time, this is what you'll want.
NSManagedObjectSubclass *myObject = [NSManagedObjectSubclass MR_findFirst];
[MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {