How to do this?

[Oddjob]

True but it's the outside screw that would be replaced not the inside ones. Saying that I replaced all of mine with stainless, saves them rusting.

[Bryanj]

Dissasemble the switch inside a large plastic bag to contain flying parts!

[paul G]

Dissasemble the switch inside a large plastic bag to contain flying parts!

I will definitely second that  Also in my experience when you are putting it back together as those springs and ball bearings are impossible to find again 
Luckily I had some knackered old switches 

[deltarider]

Unfortunately now my clutch switch cable doesn't work as before. Probably it got damaged in the process of removing and reinstalling in the clutch grip. I have temporarily removed it altogether for inspection. If I fiddle with it, it sometimes works and sometimes not. On close examination I noticed on the switch side it has a little piece sticking out as if it has to sit in a certain position. Does any of you know if that is the case? Can I repair it?

[McCabe-Thiele (Ted)]

There is a small slot in the lever unit that the small notch fits into if they are the same as a 400 unit cheap ones on bay that work.

[Oddjob]

Normally not DR, they are pretty flimsy inside I'm afraid. New one isn't dear.

[deltarider]

There is a small slot in the lever unit that the small notch fits into if they are the same as a 400 unit cheap ones on bay that work.

You're right, it's very, very difficult to detect. I've tried to make a pic using a small endoscopic camera, but even that pic doesn't show it clearly enough. Anyway, it - that little slot - is there. It faces forward. Best to use a small torch or whatever light to position the switch correctly.
@ Oddjob If it isn't pulled, it works, so I may give it another try tomorrow.

[Oddjob]

The slot on the switch is closest to the bars

[deltarider]

The slot on the switch is closest to the bars

By 'bars' you mean the handlebar, right? Are you sure? Even with glasses my eyesight is not good enough.

[Oddjob]

Just checked my newly restored switchgear and can confirm that the slot that the little tag on the clutch switch goes into actually runs all the way along the flange which holds the clutch lever. It's at 90 degrees to the housing and carries on a little past the hole where the clutch switch passes through, so either way will work, facing directly out or facing directly in. On the assumption that dirt and crud will accumulate more on the inner part of the slot I'd fit with the tag facing outwards, mines facing inward but mine had just been blasted and ceramic coated.

[McCabe-Thiele (Ted)]

When I fitted my clutch switch I just gently pulled on the wiring end then rotated the wire - when the tag aligned with the slot it then pulled in with no effort - I had cleaned the slot with some WD40 using a small bottle brush.
Once fitted I had to trim a little off the end of the plastic pluger to get it to work as it should.