As shown in this image shared by Reddit user brooksdbrewer, the option appears as an Edit button when you scroll down to the bottom of the Control Center menu. Tapping the button then allows you to drag the icons around into a new arrangement – placing the icons you most often use up to the top of the menu, for instance.
So for example, if you use Theater Mode frequently, you could locate the associated icon next to the battery percentage icon at the top, and move the Ping iPhone icon to the bottom of the menu to prevent accidental taps. Likewise, you could drop the Expel Water icon right to the bottom of the screen if you never use it.
Whether or not the Control Center's new Edit function makes it to the final public release of watchOS 5, due in the fall, remains to be seen. However, there are plenty of other features to look forward to, including Activity Competitions so you can compete on workouts with friends, Automatic Workout Detection and new Workout types, a new Walkie-Talkie app, and more.