Hi, I have a mahogany stained staircase in my open plan living room and I hate it. I would either like to lighten say to an oak colour or have it painted white. Problem is that I will not be lifting carpet. Can anyone suggest how I start this task, I am frightened in case I make a mess of it. Any advice would be greatly appreciated.
Thanks.

If it's real mahogany, you could sell it for more than the cost of a new one in oak or any wood you want. If it is just a mahogany colored stain, then I would advise that you paint it. Making it lighter will be tremendous work in all the corners and edges, and probably won't come out the way you hope anyway.
To paint, buy blue paint tape (sort of a wide masking tape that doesn't stick too much and leave glue) and tape newspapers all around the bottom of the area. Start at the top. Stir the paint well, often, in a small paint pail, not the can. Use a good quality brush. Apply sparingly to avoid drips. You may need to do 2 coats. The paint department can help you with the right choices for a staircase paint.

Protect carpet as joypulv has stated in post #2.

You will have to hand sand each spindle to open pores of wood and rough up sealer.

First coat of paint would be a good primer/stain blocker like Kilz.

Then paint with finish you want.