I have Nokia Lumia 635 with WP 8.1, version 8.10.12397.895.
When I try Phone update it states that my system is in actual state and no updates are available. I have read that Update 1 should be available already.
Why do I not get it?
8.1 Update 1 is part of the Nokia Denim update, which is set to roll out to Lumia phones during Q4 of 2014.
I looked at your profile and saw you're from the Czech Republic. Microsoft has a page showing the availability for each model per region for new updates. This page can be found here: Availability in Europe.
On this page, I didn't find any available updates for the Nokia Lumia 635.