Take two paper clips (large smooth ones work well), put the larger rounded parts under opposite corners of the keycap and gently pull up until it pops off. If it feels stiff and hard to pull off, try to wiggle it off by alternating gentle pulls on each corner. Once you get a few of them off you can probably get your fingers in there and pull off the others.
Be careful on the larger keys, depending on the type of keyboard, it can have somewhat delicate stabilizers underneath that can break if you are too rough.