K bike sticking headlight switch: You can try spraying some WD-40 into it which won't hurt anything but if you want to really fix it you need to disassemble and clean/lubricate it. It's a fair amount of work though:
I highly recommend taking it off of the bike and doing it on a workbench. There's lots of little parts to lose.
At the bottom, there's a Phillips screw that holds it to the perch. Use a good screwdriver that's well seated as it can be sticky and you can strip the head if you're not careful.
Follow it's wiring and cut the zip-tie that holds it's and other wiring to the frame. Lift the tank and disconnect the switch from the main wiring harness by pressing on the sides of the connector. Take it to your workbench/table.
Remove the turn signal button - small Phillips screw on the back of the button. Remove the horn button. Small Phillips screw in the center of the back of the switch assembly in a little square hole. It's usually easier to unscrew that one if you keep the horn button depressed.
Their are three Phillips screws that hold the back cover on. Remove those, noting which one goes where since they vary in length.
The headlight switch pivots on a small metal dowel. Using a pin or something, push that metal dowel out and the switch button will come off. The switch workings are all under the button which is why spraying some WD-40 on it from the outside will not really help. Clean and lube everything that falls out when you remove the yellow switch. WD-40, silicone, whatever. It's kind of complicated with numerous little parts in there but it only goes back together one way so if you pay attention while you're taking it apart it shouldn't be that hard to reassemble.