Initial situation

Here appears the light switch that is going to be changed. It is an old model that doesn't work properly. A permanent bad contact between the copper terminals of a switch can start a fire due to overheating of the plastic parts of a switch.

Turn off the main switch

Before start working, we must ensure that no electricity is present on the part of the installation we are working on. Go to the main electrical panel and turn off the proper switch.

Lift the frontal lid

Using a flat screwdriver, lift the frontal lid of the switch.

Disconnect the electric wires from the switch

Using the screwdriver, remove the switch from the junction box. Then disconnect the electric wires. Again use the screwdriver.

Preparing to fit the new junction box

As I commented. The old light switch was very old. Newer ones need bigger junction boxes. So in this case we must remove the old junction box and fit a new and bigger one.

Fitting the new junction box - Adjusting its dimensions

Using a waterproof marker, draw over the tiles the borders of the new junction box. Then follow the lines with a diamond tip. Be sure to worn away the tile properly.

Fitting the new junction box - Ooops!

Ooops! I didn't press intense enough the diamond tip. Then when started breaking the tile with a hammer the breakage surpassed the intended breaking line. I'll fix this later.

Fitting the new junction box - Room enlargement

Using a scarp and a hammer, continue breaking the tile and mortar in order to fit the junction box. Note that this time I reinforced the tile border with tape.

Fitting the new junction box

After removing the old junction box and enlarging the hole, prepare to fit the new one. Be sure to pass the electric wires inside the superior hole of the junction box. You may have to break a tiny plastic lid if your junction box has no hole.

Fitting the new junction box - Fixing it with silicone

Once the junction box is placed. Fix it with silicone. Silicone will allow us to remove the junction box in the future with less difficulty. If you prefer you can use mortar or plaster anyway.

Wiring the new light switch

Plug the electric wires in the new light switch. You may have to use the screwdriver again if the switch model you have chosen has no special fixations.

Removing the screws from the junction box

Using the screwdriver again, remove the screws from the junction box. Keep them.

Fixing the switch mechanism

Place the switch mechanism over the junction box. Use the screws removed on the last step to fix the switch mechanism over the junction box.

Placing the new frontal lid

Place the new frontal lid. Usually this part fits by gently pressing over it once is placed in front of the mechanism.

Final result

And that's it! Note that I have managed to fix the broken tile. The final result will be better if you have spare tiles and can replace the whole tile. But it was not my case.