Hmm, there are usually a pair of plastic holders that can be reached in a thin rectangular hole at the bottom of the cart at the edge with no connector. Insert something of the right size to make them let go and pry it open, preferably very carefully - plastic this old is usually brittle.
But wait, there are two more, they are placed on both sides of the shell inside the connector compartment.
The ones that need to be bent are red and the holders are green.
Perhaps the Mega-Cart maker(s) have some tips on how to make it easier.
On my Mega-Cart there are obvious prying marks along the edge of the rectangular holes from a rather narrow object, one would have thought when making over a hundred of these that a proper tool would be developed.
If you warm the shell up before starting to pry you may stand a better chance of not breaking the plastic tabs. I successfully opened a copy of Star Battle with aluminum label that doesn't work, almost no visible marks. If you try and use plastic tools there's less risk of leaving visible trails.
For example by putting it in hot water before opening just make sure it's completely dry before you put it back in the VIC. You can ofcourse test it without putting the shell back on (just make sure it's dry).
BTW
You can also tighten the connectors on the VIC by inserting a thin screwdriver between the plastic of the connector and the springy metal connector piece and bend slightly until it stays a little longer away from the plastic. Don't overdo it or the cart will be very hard to insert.
EDIT: The whole playfield is blue with the Mega-Cart (like the last image) on my PAL VIC with S-video cable:
