Well i fitted a new gearnob on my astra. The one uve got looks like its the appropriate type, lift up reverse. Otherwise you would be unable to select reverse.
I think most vauxhalls have the same set up.
1. Grip the gearshaft and turn the nob anitclockwise. This takes a bit of effort but it should come loose in the end.
2. Once uve poped the old one of, you'll notice a green plastic part attached to the metal of the gearshaft. The only way to get this off is by using a hacksaw. Sounds daunting but it comes off rather easily.
3. The gear nob should come with a set of universal spacer rings (for all makes and models of cars). Put on the appropriate ring, push the gear nob on and tighten with a grub screw.
Hey presto a shiney new gearnob.
Hope this tutorial helps, sorry no pictures. Its quite simple really.