Signals can be reordered in Waves by highlighting signals and dragging them with the mouse (left-click+drag) or 1-finger touch to the desired location.
You can highlight multiple signals to move by holding the CTRL/CMD key and clicking on each signal name. You can highlight a group of signals in-line by holding the SHIFT key and clicking on signal names. You can highlight all signals with CTRL+a or CMD+a. You can deselect all signals with CTRL+d.
You can reorder signals the the top or bottom of the list by right-clicking on the signal name and clicking on "re-order to top" or "re-order to bottom" on the pop-up menu.
The signal list can be sorted in alphabetical order with the sort A-Z menu option under the Tools top toolbar or the right-click signal name pop-up menu.
*Note: Signal order changes are not currently saved when you exit Waves. This is a known issue.