How to remove the upper case.
Power down your Mac mini, disconnect all of the cables, and flip it over.
Insert the Jimmy into the crack between the aluminum top housing and the plastic lower housing.
The Jimmy should reach a stop about 3/8" down.
Gently bend the Jimmy outwards to pry the crack open a little larger and lift the lower housing up a small amount.
Once you have the first side free, rotate the Mac mini and start prying up on the front edge.
Use the same prying motion to both bend the clips inward and lift the lower housing up out of the top housing.
You may need to move the Jimmy along the edge to pry up all of the clips. Be patient and do a little bit at a time.
Keep working around the perimeter, freeing the clips along the final edge.
Flip the Mac mini back over and lift the top housing off of the lower housing.