Removal pretty much consists of reversing the installation steps, but there are a few subtleties...
As was the case for installation, this procedure uses a second spout, as well as dental floss and lubrication.
Pull the installed TetherSpout forward enough to access the cross holes. Thread a length of floss through those two holes, ending up with equal lengths on each side. Then, thread both floss ends through the bore of the second spout, entering by the tapered tip and exiting at the flange end.
Bring the two spout tips in contact and tension the floss to keep them that way. Bring the assembly out as much as possible. This will drive the retainer as close to the penis tip as it can go.
Also, make sure the main spout's side holes (through which the floss is
passing) are vertical (one at the top, one at the bottom). This will be
optimal in the end, when the retainer is being pushed out, as it will minimize the retainer's tendency to flip out of alignment.
Lubricate the second spout a bit. While lightly holding the glans between two fingers to prevent the retainer from sliding backwards (but not hindering the movements of the spouts), gently push the second spout in, while maintaining tension on the floss. This will cause the first spout to start sliding further in, through the retainer:
Push the second spout all the way until its flange contacts the penis tip.
If you have held the retainer in place, it should now be clear of the first spout. Now release tension on the floss and pull the second spout out. With a bit of extra side to side finger pressure, you will feel the retainer flip and align itself flat between your fingers in the vertical plane.
