Let's get oriented.  I will refer to the side of the vacuum cleaner with on-off switch as the top.  The side where one inserts the hose is the front.

On the top, behind the hose, there is a slot that you can put your fingers in.  I insert the blade of a large screwdriver here, with the handle pointing toward the front.  I gently push the screwdriver down.  The screwdriver is a lever, and the front of the slot is the fulcrum.  As I push the front down, the blade pushes up the plastic cover in back of the slot.  This cover is retained by two clips.  With enough force, they will release and the cover will swivel up and can be completely removed.  Then the bag can be replaced.

Please be careful when you try this not to damage the plastic cover.