Comcast Xfinity has a fairly new package that includes a Netflix subscription. This means that instead of paying Netflix for a monthly subscription, the cost is included in your Comcast Xfinity package, and billed through Comcast Xfinity.
If you want to upgrade your subscription from HD to UHD (4K), you'll get a message on the Comcast Xfinity page to give them a call to change your package.
Don't do that.
Instead, sign into your Netflix account on the Netflix website, and upgrade there. You'll be charged through your Comcast Xfinity billing process.