How to change Shimano Revoshift cable?

[HBAndrew]

Hi, the cable for my back gears has snapped, and I need to get into the Shimano Revoshift to replace it but I just cant figure it out, I removed a little screw from the top to see if I could get it open but it only comes off slightly but I cant seem to get it right off with fear of breaking it, can anyone help?

[HBAndrew]

Here is a picture of it, I took the top screw out where it says shimano, but theres something holding the bottom part on and I cant find what.

http://www.shimano-europe.com/cycling/phpimages/product/picture/slrs41ar.jpg

Thanks