It's funny this question was ask because I asked the exact same question yesterday at Best Buy as I spoke to a Samsung rep and got the same answer.... press the red end call icon. That said, for FYI...
Asking to demonstrate, he called my phone and we found the red hang up icon didn't appear. The reason.... I'm using a Bluetooth earpiece paired to the phone, which in turn causes the phone itself to handle calls, and thus I either end the call via the phone or earpiece. I can answer calls from the watch, I just can't end them if I'm using my BT earpiece.
But yeah, the red icon does appear (if no BT earpiece is used) and does work. Tested that as well on my 46mm Galaxy Watch.
Yet another Galaxy Watch quirk.