I've got a set of Reba Team Air forks from 2007. They are set at 100mm at the moment. Been told I can change the travel. How do I do this?

First have a look at the service manual here.(just after step 10)

There is a plastic spacer you can remove, but it does mean dismantling your forks, or you could get them serviced and have them remove the spacer for you.