It depends on your desktop environment (KDE, Gnome, XFCE, fluxbox etc.). Google will have answers for each one, so do a quick search and see
In KDE (which is what I have to hand) you'd open the Control Centre via the K-menu > Settings, then go to Regional and Accessibility, then go to Keyboard Layout, and select the appropriate keyboard layout there. I imagine it's not that different in Gnome.
You can also change it for your ttys using the loadkeys command, although I've never used it, since I always select a UK keyboard layout during installation