Using a nylon spudger, or with care a small flat screwdriver, ease the rear cover away from the main body of the iron on both sides, starting at the top near the cable entry.

Once the rear cover starts to come away you can increase the gap created by inserting the spudger a little further down.

Note the tab on the rear cover shown in the 3rd photo which was holding the rear cover in place. On reassembly this can't be re-inserted without lifting the top cover (see Step 7 later).