How can I put the derailleur on left side?

[youngoldbloke]

Not if the wheel is being 'driven' in reverse - ie the bike goes backwards. Or have I got 'lost'?

I didn't think that was the intention. I think forward motion was required.

 

[Illaveago]

How about turning the saddle round, and tying a couple of bits of string to the handlebars for rear wheel steering. (I just did that in my head. I fell off).

A simpler method may be to remove the front forks and invert them. You will then be able to invert the cycle which will put the derailleur on the opposite side . You will need to clip your feet into the pedals and may need to fit rollers to your helmet. Riding in this position could cause blood to rush to your head.