You need to buy a tiny screwdriver which is perfect for those little screws in your glasses, then tighten all the screws in the hinges that hold the temples to the front of the glasses, then your glasses has become smaller. You can bring them from where you bought them or some sunglasses stores, they know what to do.
Really, it seems that your sunglasses is not in good conditions. Maybe you have used them for a long period of time and actually every pair of sunglasses will show signs of long use. If them are too large for you, then I suggest that you replace a pair since there are natural ways to fix it, unless you send them for mending. Maybe you just need a new pair.
If it seems a little big for you, you could use the screwdriver to tight the screw in the frame side and the nose pad. You could adjust the position of the nose pad to make it tight. Then after adjusting it, you could wear to see whether it is suitable for you. And you will see whether it will slip off of your nose again.