From a glance it seems an easy task with a sideward force would do  the job but in reality it`s rather difficult to have the board disengaged from the positioning-mounting clips.Is there any special trick to have the job done in a easy way?

Well, that is it. Some equal pressure on the board inwards and then a pull to the outside. There should be six "engagements" -pairs on top, center, and bottom. When putting back together -watch the alignment of the pin connectors towards the inside of the machine.