Here's an easy way:
Warm the can by leaving it in the hot sun for a while or with a hair dryer, etc. When the metal is good and warm to the touch, open the can and then close it and latch it securely.
Now take it inside into the air conditioning where it's cooler and let the can cool. You could even put the can into your freezer for a while.
After the can has cooled, simply open it again. The cooling will have created a vacuum in the can. If your can is airtight, you will hear a hissing sound as you break the gasket seal and air rushes into the can.
This is also an easy way to vacuum seal your stored ammo if you are so inclined.