Use a spudger to pry along the seam between top and bottom of the mouse.

Gently separate the top and bottom case until you can see the internals of the mouse.

Do not fully remove the top cover as it is connected by a cable and will damage the mouse.

Use the point of a spudger to push on alternating sides of the connector to "walk" it out of its socket and remove the cable attaching the top and bottom of the mouse.