The warp group has a warp icon with drop down selections to invert, mirror and reverse.
Warp Invert - no warp threads selected
If no area is selected, the entire warp will be inverted. In the following example, this is the warp prior to selecting Warp Invert.
Warp Invert - with area selected
If a region is selected, then only those selected threads are inverted.
Warp Mirror Selected
Select any threads within the warp to mirror. Notice that the mirroring overwrites any existing threads. If this is not desired, then insert the number of threads needed prior to mirroring.
Warp Mirror All
To mirror all the warp threads, click mirror all.