I have steam on my windows 7 PC and I knew about the new UI for a while, yet I still have the old UI. Even when I check for new steam updates it say that steam is up to date. I also can't completely uninstall steam since I have many games installed. All I want to know is how to update steam to the new UI.
The update which included a minor change to the Steam UI was delivered via the normal Steam update. In the unlikely case that you have not yet received yet this update, go to your Steam Client and run the following menu entry while being connected to the internet:
Also, make sure that you do not have any custom skins activated:
After updating your Steam Client, you should have the new Steam Skin. If all that fails, shut down Steam, delete everything except Steam.exe and the folder userdata and steamapps. After that launch Steam.exe again and it should update regulary.