To change the order that a timer appears in your list, tap and hold a timer row, then drag it to the desired position.

In the Notification Center widget, timers will not follow the order shown in the app. Instead, they will appear in the order of most recent use, with the most recent at the top. This is because only a few timers can show in the widget (a limit of iOS), and it is important that they 1) they include the currently-running timer and 2) they are relevant.

Also on the Apple Watch, timers appear in the order of most recent use. Again with the limited screen area on the watch, it is important that recently-used timers be most accessible to avoid excessive scrolling on the small screen.