In case US Cellular and Samsung are yet to push the big Marshmallow update to your Galaxy S5, you can take our little bit of help to upgrade your Galaxy S5 to Marshmallow right away.
What you can do is download the Android 6.0 Marshmallow update for your US Cellular Galaxy S5 from below, and install it all by yourself using our guide for the purpose.
The US Cellular Galaxy S5 Marshmallow update comes with build no. G900R4VXU2CPD2. Or PD2 in short. Once installed, your device will running the latest Android OS, dubbed as Marshmallow, on the Galaxy S5.
Click the link above to find the Marshmallow update for your device on our firmware download page that also contains guide to install the firmware.
In case you need any help with download and installation, feel free to ask via the comments section below.