Very easily. Unlike Android, Apple has always allowed older iPhones1 to update to the newer iOS versions using the built-in app store update process. When I turned on my phone this morning, it had a notification dialog saying something like "iOS 8 is available. Do you want to update now?"
1 Older models up to a point (3-4 versions back). The original iPhone, for example, doesn't get iOS 8 today like the 4's, 5's, and 6's.
1 Older models up to a point (3-4 versions back). The original iPhone, for example, doesn't get iOS 8 today like the 4's, 5's, and 6's.