Ultrabooks and notebooks and laptops before them have always had limited upgrade capability compared to desktops.
Notebooks and laptops were typically limited to hard disk replacements, memory upgrades, optical drive upgrades, though this usually relied on the notebook manufacturer offering a suitably packaged drive. In some cases, the wireless NIC could be replaced and many had PCMCIA card slots. All these things take space and add bulk to the machine and make it less portable. I lugged a Compaq SLT386 around for some years (about the size of a J-Block!).
Ultrabooks were developed to address the need for better portability and so the bulk and weight had to go. No more PCMCIA, Memory soldered to the motherboard, instead of SODIMMs, NIC chips similarly soldered and no optical drives. Even notebooks are losing most of these things. So all you are really left with that could be regarded as user replaceable is the hard drive, assuming the manufacturer provides access to it.
Expansion or enhancement on ultrabooks relies on USB3 ports, which can address most things except memory.
That said, if you know what you are doing, you can obtain a hardware maintenance manual which will detail how to completely dismantle the machine - but I don't advise it!