Depending on the type you have, I have two different answers.
If you are talking about pressing the actual chrome plug down, these are one of the worst plugs ever made, they often get stuck in the closed position, my advice would be to replace them with a more standard type. I prefer the swi flip over type plugs.
On the other hand if you have to press a separate lever usually found on the back of the tap fitting. These can usually easily be adjusted, but there are more than one type.
1. If you can just lift the plug straight up and out you should have a brass screw with a locking nut, just loosen the nut and adjust accordingly.
2. If you have a plug that cant be pulled out, you will need to remove the lever where it enters the back of the waste, below and behind the basin, just undo the, usually knurled chrome nut and the lever will come out, and you will see, usually a white nylon ball and some sort of rubber seal, be careful not to lose the seal. Once removed you can then lift the plug out and adjust accordingly.
Mind you, I have been a plumber for 36 years and have come across at least 10 different types, so good luck.