There is a slot underneath, probably right hand side, into which you slide up a screwdriver to unhook a catch & then can pivot up the controller {pull bottom out & up while keeping top in place} & then free it from its backing plate - be sure to turn off mains electricity first! There is a battery on the back which, when new, has a 7 day memory - so remove & replace. When controller is replaced in the cradle {hook in at top, then pivot down} all should be well. Note battery should be replaced every 5 years.
Michael Baker, June 2008