Here s how to do it without breaking the plaster and lath.
Old style light switch wiring.
Moiak light toggle switch retro single control switchs 6a round lamp switch mounted wall light button switch old fashioned toggle switch 6 26 6.
Probe the existing hole with a keyhole saw to find the horizontal edges of the lath.
Methodically dealing with one wire at a time will help avoid confusion and mistakes.
In box 1 disconnect a wire on the old switch and attach it to the correct terminal on the new one.
We need to replace it as its bust basically.
Conventional 3 way wiring is not very complex with conventional wiring the line line voltage connects to the common of one switch.
Bring old light fixtures wired with knob and tube wiring up to code by installing an electrical box in the plaster wall.
Anyway i buy a double 2 way switch to replace it picture below anyway i take off the.
Upgrading to a new rocker style switch takes just a few minutes and requires no extra wiring or special connections.
If the new switch s cover plate is smaller than the original you can make it easy on yourself and cover the old plate s outline with a slightly oversize new plate no wall repair or touch up paint required.