There’s a setting (under developer options I think) to show advance update channels. Once you’ve turned it on, you can choose different update levels for both Steam OS and the steam client. The most cutting edge version of SteamOS is main, which will put you on 3.5. This has several advantages (including better shader handling and SMT being worth using in emulated games).

However, updating to the bleeding edge software update is not without downsides. Expect things to break fairly often. I tried the main update channel for awhile, decided too much was broken, and then found that something else that was broken was downgrading back to stable or beta. I ended up being stuck for awhile until the next update came out, at which point I was able to successfully downgrade. So proceed with caution.

If you go into the developer settings area (Can’t remember what it’s called) you can turn on extra development sources, in which case you can trunk-level daily updates.