A Swiss Army knife usually has a tool that is a combination of a can opener and a small flathead screwdriver. It is simple to open a can with the can opener by piercing the can with the tool's sharp point and cutting around the inside of the can's rim with its blade. Opening a can with a Swiss Army knife can opener creates extremely sharp edges so it must be done very carefully. The Swiss Army knife can opener will not make a smooth cut like a rotary can opener, so the edges may also be jagged.
Place your finger nail in the grove along the exposed edge of the knife and lift the tool out by pivoting it out of the handle until it will not move any more. The tool and the handle should make a straight line. If the tool is an angle to the handle it is not out far enough.
Be certain that you have opened the can opener tool. It will have a small flathead screwdriver on the end. If you hold the handle with the screwdriver end pointing to the sky you will see a slightly rounded, triangular pointed blade and beneath that a u-shaped blunt hook.
Hold the can firmly on a flat, non-slippery surface with your left hand and the Swiss Army knife in your right hand. If you are left-handed, reverse this.
Place the can opener over the top rim of the can so that the triangular pointed part is on the inside of the can's rim and the blunt hook part and handle are on the outside of the rim. Pivot the tool so that the screwdriver end is facing away from you and is pressing against the inside of the rim, with the blunt hook catching the rim on the outside of the can. Press the screwdriver end towards the inner rim and the hook towards the outer rim of the can and hold the tool in that position.
Maintaining even pressure and a firm grip on the handle, press the sharp point of the triangular blade down into the can, piercing the flat top of the can close to the inside of its rim and making a short cut. The hook part of the can opener should still be catching the outside of the rim.
Lift the end of the can opener so that the knife is horizontal, keeping the rim of the can between the blade and hook parts of the tool. With your left hand move the can about half an inch clockwise while holding the knife steady, so that the point of the triangular blade rests at the end of the slash you just made. Press the blade down and away from you again, cutting through the top of the can. This should not make a new hole, but continue slicing through the flat top in one continuous circle.
Repeat this motion, continuing to cut the top of the can close to the inner edge of its rim along the same cut line until you have gone almost all the way around the can. When it looks like it would require one more cut to complete the full circle, stop.
Use the screwdriver end of the can opener to lift up the top of the can with the small intact section of the top acting as a hinge. Both the rim and the cut edge of the top of the can will be extremely sharp so be very careful when pouring out the contents of the can.