Make sure you turn the power off to the light switch at your consumer unit.

Unscrew the two retaining screws on the switch and pull away the switch.

Tip If it's an old switch that has been painted around many times over the years a good idea is to score round it with a sharp knife. This stops you pulling away the paint work on the walls.

Check that there is no live with your voltage checker, having previously checked that the voltage checker is actually working.
(see my section on voltage checker).

You will see differing amounts or cable depending on what type of switch it is and how it has been wired.
You'll need to replace like with like and replace the cables in the same configuration. Make a note of what goes where and mark them with tape before you unscrew the cables. When you screw the cable to the new switch make sure they are nice and tight and no bare wire are showing. Re-fix the switch to the back box and turn power back on.

NB Metal switches must be earthed, and usually come supplied with an earth lead. This should be connected to the back box with the other earth cables that form part of the wiring. If you're replacing a plastic switch with a metal one and there is no earth to connect to call a qualified electrician.

A one way switch has a common (C) and Line out (L1) terminal.
The common is the main live coming in. The L1 is the switched live going out to the light.They will be marked so you can tell which is which.

A two way switch (for switching a light from two different locations like in a hallway) has a common (C) a L1 and an L2 terminal. L1 is off when L2 is on and vice versa depending on the switch position.

You can use a two way switch for one way switching. Just ignore the L2 terminal. Some switches will actually come two way as standard for use as both.

An Intermediate switch(UK light switch) is used for switching a light from 3 or more locations in conjunction with two way switches.
They have 4 terminals L1, L2, L3, L4.

NB In light switches you are actually switching the live part of your circuit only so all of the cables(except the earths) even if they are different colours are actually part of the live not neutral. This may confuse you as the colours may be red and black for older cables or brown and blue if they are the newly introduced (post april 2006). This has been done for convenience when your wiring was done. If done properly the cables should have been clearly marked with colour coded sleeving as live. (red or brown)

Any bare earth cables should have yellow and green earth sleeving.