It's simple to open a cantilever brake to remove a wheel or dismount the cable.
Cantilever brake connections
One end of the straddle wire on a cantilever brake is clamped, the other is held by a nipple.
Push the cantilever brake arm holding the nipple-end of the straddle wire against the rim.
The other side
Pull the nipple in the other direction and unhook the cable through the slot. The cantilever brake is now open.