Not too hard.
Machine should be empty (of water - spin it out if you have to). Turn it to lying partly on its side - say 45 degrees to the left, as you face it. Mains electicity off. Reach inside through the hole in the bottom. Find the black cable which controls the latch. It loops down from the top of the machine and back up to under the latch. Move the whole cable upwards, so it pushes towards the latch. Actuate the door latch, preferably meanwhile. Door should open.
There is a "paddle " device at the top of the machine, into which this cable runs. If it is not working properly, the door will always not open. Its mysteries are not always clear to me. They could be the subject of a separate essay
A S Beesley, August 2007